Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is end-of-sentence punctuation. This choice correctly uses a period to punctuate a declarative sentence ending with a relative clause that presents an indirect question (“why ganga has been used as a communication method.”)
Why the other choices are wrong
Choice A
Choice A is incorrect. It’s unconventional to use a question mark to punctuate a declarative sentence that presents an indirect question.
Choice C
Choice C is incorrect. The structure requires a declarative clause at the end of the sentence, not an interrogative clause that asks a direct question, such as “has ganga been used as a communication method.”
Choice D
Choice D is incorrect. The structure requires a declarative clause at the end of the sentence, not an interrogative clause that asks a direct question, such as “has ganga been used as a communication method.”