Keep in mind that in these cases, the word "pair" becomes the subject of the sentence (see http://grammar.ccc.commnet.edu/grammar/sv_agr.htm), so making the substitution in (a) would mean that the plural subject "pairs" would take the plural verb "look" instead of the singular verb "looks."
Since the word "this" is only used with singulars, and the word "these" is only used with plurals, both (b) and (c) are automatically incorrect and can be eliminated.
That leaves (d), but to me (d) would feel much better if "looks" were substituted for "look." That way, a singular subject would be taking a singular verb.
Can you provide any background on this question? Is it part of a standardized test of English?
[not a teacher]
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