Let’s examine the sentence: “Don't mention it. It's the ____ I can do.” There are two sentence. The first one insists on not mentioning whatever one has done. We can assume that the speaker considers it not big enough to be mentioned. Hence, the second sentence requires the superlative of ‘less’ or ‘little,’ which is ‘least.’ We use the least with a noun to mean ‘less than all of the others’. Hence, Option B is the correct answer. All the other options would make the sentence meaningless.