Both are correct. In B, I would insert "the": "complete the sentence by using the present perfect tense." The only difference between A and B is that "by" makes it more specific, and inserting an extra "the" would also make it more specific.

Both options A and B are grammatically correct, but they have slightly different meanings and uses: A. "Complete the sentence USING Present Perfect Tense!" In this sentence, you're instructing someone to finish a sentence, and the means or method you want them to use is the Present Perfect Tense. This is a straightforward directive. B. "Complete the sentence BY USING Present Perfect Tense!" In this sentence, you're also instructing someone to finish a sentence using the Present Perfect Tense, but you're emphasizing that the Present Perfect Tense is the tool or mechanism they should use to achieve this. It implies a bit more of a causal relationship between the action (completing the sentence) and the means (Present Perfect Tense). So, it depends on the nuance you want to convey. Option B is slightly more explicit about the method to be used, but both options are acceptable in most contexts.