This sentence as written contains an error: When two things (or people in this case) are compared, the correct form of the word is “better,” not “best.”
Eliminate choice (A).
In the answer choices, there is a 3-2 split between “whether” and “if.” Because two people are being compared, “whether” is correct. “If” is generally not used in comparisons but only for the conditional, as in “If Roger Federer wins another Grand Slam title, he will be the greatest tennis player of all time.”
Eliminate choices (B) and (E) on this basis. (For the record, there are other problems with these two choices as well. Choice (B) contains the same error (“best”) as the original. And by changing the verb at the end from “should be” to “will be,” choice (E) alters the meaning of the sentence.)
Read choices (C) and (D) in parallel. They are alike until near the end, where (C) uses the pronoun “they” and (D) uses “either.” The subject of the clause is “Roger Federer or Rafael Nadal,” and because of the “or” and because each noun in the compound subject is singular, verbs and pronouns relating to the subject should be singular. The verb “is” is correctly used in both choices, but the plural “they” is incorrect in (C). Additionally, choice (C) uses the future perfect verb tense “will have been,” which alters the meaning of the sentence.
Answer choice (D) is correct.