Which is the closest synonym for the word solidify, as it is used in the article?

The closest synonym for the word “solidify” in this context is A. strengthen.

Explanation: “Solidify” generally means to make something more definite or stronger. In many contexts, it refers to the process of making ideas or plans more concrete or firm, which aligns closely with “strengthen.” The other options do not convey a similar meaning to that of “solidify.”