Which choice completes the text so that it conforms to the conventions of Standard English?
A
instrument:
B
instrument
C
instrument—
D
instrument,
Explanation
Correct answer: B.
Choice B is the best answer. The convention being tested is punctuation use between a subject and a verb. When, as in this case, a subject ("the strings inside the instrument") is immediately followed by a main verb ("are plucked"), no punctuation is needed.
Why the other choices are wrong
Choice A
Choice A is incorrect because no punctuation is needed between the subject and the verb.
Choice C
Choice C is incorrect because no punctuation is needed between the subject and the verb.
Choice D
Choice D is incorrect because no punctuation is needed between the subject and the verb.