Answer: A. requirement
Step-by-step explanation:
Apart from its meaning of a grammatical mood which expresses an order, the word 'imperative' also has the following meaning - an essential action, a requirement. We often use this word to refer to something that must be done and should not be avoided. For instance - "Visiting the Niagara Falls is an imperative!" or "It is an imperative that I speak to him right now."