Answer: The right answer is the A) conoce.
Explanation: Just to elaborate a little on the answer, it can be added that in this exercise they are asking you to fill the blank with the correct form of the verb and the most appropriate verb. The verbs "conocer" and "saber" can translate as to know, but they are used for different purposes in Spanish. "Saber" could be broadly translated as to know about, whereas "conocer" could be broadly translated simply as to know. In this sentence, the speaker is saying that his or her brother (third person singular) has been to the United States. Options C and D are incorrect, since they agree with the third person plural. If he knew about the US, the speaker would have said "sabe de" (knows about, knows of), so the right option is "conoce."
