"Sullen" means gloomy or depressed, so "gloomy," an adjective that means murky or glum, is the correct answer. "Cheerful" is an antonym of "sullen"; it means extremely happy or joyful. None of the other answer choices are close in meaning to "sullen": "peaceful" means calm or tranquil; "optimistic" means hopeful or having a positive attitude; and "fluid" means free-flowing, or effortless.

"Ardent" means enthusiastic or passionate towards some cause. "Aquatic" refers to water and its characteristics. "Barren" means lifeless or unable to produce crops. "Sincere" means honest or without deceit. "Impulsive" means sudden and without thought or planning.

"Keen" means to be eager or excited for something. "Confused" means incapable of thinking straight or clearly. "Despicable" means being hated. "Contentious" means argumentative or quarrelsome. "Indecisive" means incapable of deciding issues in a timely manner.

"Acrid" means biting or bitter. Therefore, "bitter" is the best synonym. 

"Woeful" means full of woe, or very sad, so "morose," an adjective that means miserable, glum, or irritable, is the correct answer. "Courageous" means brave. "Stalwart" means strong and reliable. "Destitute" means poor and impoverished. "Shabby" means run-down or in a very bad condition.

“Poignant” means emotionally painful. It can also mean to the point or piercing. So, "moving," which can mean evoking emotion, is the correct answer. None of the other answers are close in meaning to “poignant”. “Developed” means matured or progressed. “Green” is the color of grass. “Friendly” means kind or welcoming. “Listless” means lacking energy.  

"Giddy" means excitable or frivolous. Similarly, the correct answer "flighty" means easily excited or lacking stability. The other choices do not have similar definitions: "accelerating" means speeding up; "young" means juvenile; "stable" means not often changing; and "destroyed" means spoiled, wrecked, or defeated.

"Placid" is an adjective that means not easily perturbed or excited when referring to a sentient living thing; it can also mean peaceful and calm, and is often used this way to describe bodies of water. While "dull" might seem like a good potential synonym for "placid," "dull" actually means boring or not sharp, so it doesn't mean the same thing that "placid" does. "Tranquil," however, means calm, and because it is the answer choice closest in meaning to "placid," it is the correct answer.

"Nonplussed" means surprised and bewildered, so "surprised" is the correct answer.

If you are “aghast," you are horrified, so "horrified" is the correct answer. For clarification, "cerebral" means intelligent or brainy; "jubilant" means very happy; "scornful" means negative and insulting; and "ghostly" means spectral or like a phantom.