Which choice completes the text so that it conforms to the conventions of Standard English?
A
are
B
have been
C
are being
D
is
Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is subject-verb agreement. The singular verb "is credited" agrees in number with the singular subject "mathematician Grigori Perelman."
Why the other choices are wrong
Choice A
Choice A is incorrect because the plural verb "are credited" doesn’t agree in number with the singular subject "mathematician Grigori Perelman."
Choice B
Choice B is incorrect because the plural verb "have been credited" doesn’t agree in number with the singular subject "mathematician Grigori Perelman."
Choice C
Choice C is incorrect because the plural verb "are being credited" doesn’t agree in number with the singular subject "mathematician Grigori Perelman."