Something that is "strenuous" is very challenging, whether it be physically or mentally challenging. Thus, "difficult" is the best synonym because something that is "difficult" is not easy to complete. Thus, "strenuous" and "difficult" are synonyms. 

Something that is "toxic" is dangerous because it is a substance that is not tolerated by most organisms, especially the human body. If someone consumes something toxic, that person could get very sick or even die. Something that is "poisonous" has the ability to make an organism very sick or kill it. Given that "poisonous" and "toxic" are synonyms, "poisonous" is the correct answer.

If somewhere is “arid” that means it is very dry and does not get very much rain. So the desert is very “arid.”

The word “rigorous” means hard, challenging, or difficult. It is the opposite of “easy.” “Serene” means quiet and peaceful, so it is the opposite of “violent” and “loud” for different reasons.

When someone feels weary, they are tired and fatigued.

The correct answer is "drowsy," which means groggy or sleepy. "Lively," "energetic," and "active" are all opposites of "tired."  "Drowsy" is the only word that means nearly the same thing as "tired."

The word "listless" means having little energy or interest in things. The best choice is "lifeless," which means without energy. Considering the other choices, "unwritten" means not recorded in writing, "wandering" means moving from place to place without a destination, and "forbidden" means not allowed.

"Tired" is an adjective that means lacking energy or needing to sleep. So, the answer choice closest in meaning to "tired" is "exhausted," an adjective that means drained of bodily resources or tired. None of the other answers are close in meaning to "tired": "hungry" means needing food; "young" means not very old; and "embellished" means having details or additional components that make something more attractive.

"Drowsy" means sleepy and exhausted, so "tired" is the correct answer. "Excited" means very eager to do something or interested in something. "Painful" means causing pain or suffering. "Concise" means short and to the point. "Agile" means nimble or able to move swiftly.

"Dormant" is an adjective that means "not doing anything at this time : not active but able to become active," or "having normal physical functions suspended or slowed down for a period of time; in or as if in a deep sleep." So, neither "awake" nor "energetic" can be the correct answer because each is an antonym of "dormant," not a synonym. "Inactive" is the closest synonym to "dormant" amongst the listed answer choices, so "inactive" is the correct answer.