In the question given below, a sentence is given with

Solution

In A, we need a verb in the present tense that is compatible with the modifier SHOULD. The sentence talks about how people need to understand that the environment needs to be protected. In B, we need a noun that completes the idiomatic expression AT THE COST/EXPENSE OF something. The sentence talks about how development cannot come at the expense of the environment. COSTLY is an adjective and is hence inapplicable here. In C, we need a need a preposition that is compatible with the verb LEAD and the object PATH. The sentence talks about how environmental destruction may cause humans to go extinct. UNDER is incorrect here because LEAD DOWN THE PATH is the correct usage. Hence, (a) is the correct answer.