Which of the following describes an allele whose characteristic phenotype is masked by the presence of a second, different allele?

Do need a little more info there bud, 

but if you're looking up this kind of question, may I suggest looking up "Dominant" and "Recessive" alleles-- 

Dominant alleles are usually the ones who's phenotype is being shown-- recessive pretty much only show themselves when there are two of them (as in humans, we only pretty much have two of each in our DNA)

Think of genotype as the blueprint, the phenotype is the house itself

The Dominant allele is what the boss wants, the recessive allele is what the worker wants

If there are two bosses (two dominant alleles), they both get what they want
(which could be a case of either incomplete dominance or co-dominance)

If there is one boss, one worker-- the boss gets what he wants still-- he's the boss (the dominant allele overshadows the recessive)

If there are two workers, and the two workers want the same thing (and there's no boss around) then they get what they want (two recessive alleles expressing phenotype)