Answer: Cytosine and deoxyribose sugar
Explanation: : Remember the base pairing rules: cytosine with guanine and adenine with thymine, in equal amounts. To compliment the given base,, you'd have to choose the nucleotide that would pair with it, attached to a phosphate-sugar backbone. I'm not quite sure why they didn't include phosphate in that option, but it's the only possible one. Guanine couldn't pair with any of the other listed options.
Hope this helps,
Lacia :)