The correct answer is:
B. two tailed
In a two-tailed test, you are checking for differences in both directions (i.e., whether μ_1 is either greater than or less than μ_2). Since your alternative hypothesis (H_1) states that μ_1 is not equal to μ_2, it indicates that you are looking for a difference in both directions, which makes it a two-tailed test.