The correct answer is A. Briskness.
Alacrity is a feeling of eagerness and enthusiasm. It is often used to describe someone who is quick and cheerful in their actions. Briskness is also a feeling of eagerness and enthusiasm, but it is more specifically used to describe someone who is quick and efficient in their actions.
Fear is a feeling of anxiety or dread. It is often used to describe someone who is afraid of something or someone. Frankness is a feeling of honesty and openness. It is often used to describe someone who is direct and truthful in their speech. Alarm is a feeling of sudden fear or anxiety. It is often used to describe someone who is startled or frightened.
In the context of the question, alacrity is the most appropriate word to use. This is because the question is asking for a word that describes someone who is quick and cheerful in their actions. Briskness is the closest match to this definition, as it describes someone who is quick and efficient in their actions. However, briskness does not necessarily imply cheerfulness, so it is not the best answer to the question. Fear, frankness, and alarm are all words that describe negative emotions, so they are not appropriate answers to the question.