Without a doubt, the widespread use of renewable energy is a key solution to climate change. However, it is not a ___________, as efforts in conservation are equally crucial.

The four choices given to us are phrases. We look at the meaning of each phrase individually 

(A) Dead ringer: This means an "exact lookalike",; which is irrelevant to the implied meaning here.
(B) Silver lining: This refers to a positive aspect of a bad situation , which also doesn't fit the context. 
(C) Red herring: This phrase is use dto refer to a distraction from a real issue, which is not the intended meaning in the sentence, and hence cant be used here. 
(D)  Silver bullet: This refers to a simple solution to a complex problem. The sentence argues that renewable energy alone cannot solve climate change—it’s not a one-step fix. Hence, Silver bullet fits perfectly to the context here, and is the most appropriate choice. 

Correct answer (D).