The blank (B) must be filled with a word that fits both grammatically and contextually. It refers to a time period when it rained continuously for millions of years, so we need a noun that expresses a large or significant span of time.
(a) era (noun): Correct. "Era" refers to a significant period of time in history or geology, which fits both grammatically and contextually. It accurately describes the prolonged period of rain in the passage.
(b) rapid (adjective): Grammatically incorrect. Contextually wrong as well, since it describes speed, not a period of time.
(c) perform (verb): Grammatically incorrect. Also contextually irrelevant, as "perform" refers to carrying out an action, not to a duration of time.
(d) vision (noun): Grammatically acceptable, but contextually incorrect. While it is a noun, "vision" refers to sight or foresight, not to a time span or historical/geological period.
(e) loud (adjective): Grammatically incorrect - an adjective cannot function as the object of an article. Contextually irrelevant - "loud" describes sound, not time.
Only (a) "era" is both grammatically and contextually appropriate to complete the sentence.
Hence, option (a) is the correct answer.