Explanation
Correct answer: D.
Choice D is the best answer. “Sometimes” logically signals that the information in this sentence—that words retained much of their original sound—describes an occasional exception, elaborating on the previous statement that the evolution of words to sound very different from their proto-language origins “wasn’t always the case.”
Why the other choices are wrong
Choice A
Choice A is incorrect because “however” illogically signals that the information about words retaining much of their original sound contrasts with the previous statement. Instead, the sentence describes an occasional exception, elaborating on the previous statement that the evolution of words to sound very different from their proto-language origins “wasn’t always the case.”
Choice B
Choice B is incorrect because “moreover” illogically signals that the information about words retaining much of their original sound merely adds to the previous statement. Instead, the sentence describes an occasional exception, elaborating on the previous statement that the evolution of words to sound very different from their proto-language origins “wasn’t always the case.”
Choice C
Choice C is incorrect because “thus” illogically signals that the information about words retaining much of their original sound is a result or consequence of the previous statement. Instead, the sentence describes an occasional exception, elaborating on the previous statement that the evolution of words to sound very different from their proto-language origins “wasn’t always the case.”