OG详解-OG10 文法 Q7

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.” 
Choice A is incorrect because the plural verb “are credited” doesn’t agree in number with the singular subject “mathematician Grigori Perelman.” 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 is incorrect because the plural verb “are being credited” doesn’t agree in number with the singular subject “mathematician Grigori Perelman.”