In the conventional stage of moral development, right vs. wrong is based upon societal conventions and expectations. Since Cody stated something that is a social convention, rather than a personal ethical belief, he is demonstrating features of the conventional stage of devlopment.

At stage three of Kohlberg's theory, which usually begins in adolescence or adulthood, people become conscious of how the society at large views their behavior. Thus, they are not necessarily concerned with gaining a direct reward or avoiding punishments, but rather focus on cultivating behaviors like appreciation and good manners that will make them well-liked by society. This stage is defined by conformity to societal expectations of behavior.  

Moral reasoning at the preconventional level focuses on punishment, rewards, and authority. The child in this question is exhibiting this degree of moral consideration.

Moral reasoning at the conventional level focuses on social order, upholding the law, and maintaining positive relationships. 

Moral reasoning at the postconventional level focuses on principles and values beyond laws that ensure individual rights. 

There is no unconventional level of moral reasoning, as presented by Kohlberg. 

Kohlberg's three stages are preconventional (in which morality is determined by the consequences for the person), conventional (in which morality is determined by social rules), and postconventional (in which morality is determined by core values). Since Elliott is choosing to keep the Earth clean not because he will get a fine or because his friends will judge him, but because of his own moral compass, this is an example of the postconventional stage. 

Lawrence Kohlberg is arguably the most influential psychologist in the field of moral development, particularly in children. He posits that humans can fall into three different stages of morality: preconventional, conventional, and postconventional morality. He used paradigms like the Heinz dilemma to assess children's responses, and determine which level of morality they possess. 

Also known as “identity crisis,” individuals in their late teens and early twenties struggle with discovering their identity. As these individuals begin venturing into the world, they struggle with who they are or want to be and their purpose in the world.

James Marcia proposed the Identity Status Theory based on Erikson’s stage of identity versus role confusion. His theory led to four different results: diffusion, foreclosure, moratorium, and achievement. Individuals drifting through life with no purpose or intention for exploration are in a state of diffusion. Individuals who have explored and discovered their purpose and identity are in a state of achievement. 

Piaget argued that children under the age of ten view the world in terms of Moral Realism, believing in fixed, universal rules and judging actions by their consequences, rather than the intent of the actor. After ten, he claimed, children learn that rules are socially created things that are adjusted as needed and begin to judge actions by the intent of their actors as they were done.

The Heinz Dilemma refers to a story used by Kohlberg in his study on moral development about a man who steals drugs to help his wife who is dying of cancer. By asking how people would react to this dilemma, Kohlberg developed his stages of moral development which correspond more or less to peoples' age groups.

Pre-conventional morality is most common amongst infants or toddlers whose primary focus is to avoid punishment and seek reward. Consequently, any morals they have will be based on that principle.

Kohlberg's stages of moral development were based on Jean Piaget's stages of childhood development. Kohlberg's first stage is the Pre-Conventional, which consists of obedience orientation (punishment avoidance) and self-interest orientation. The second stage is the Conventional, which consists of interpersonal conformity to social norms and authority orientation. The third and final stage is the Post-Conventional, which consists of universal ethical principles (conscience) and social contract thinking.

Kohlberg's three levels of moral thinking include preconventional, conventional, and postconventional. 

From birth to about nine years of age, a child is believed to experience what Kohlberg called the preconventional phase. At this time the focus is self-interest; the child will follow rules just to avoid punishment and achieve a concrete reward. 

The other two options, “preoperational” and “concrete operational” are incorrect because they are stages from Piaget's theory of cognitive development.