The correct answer is B. The bird flew over my head.
The other options are incorrect because they do not describe the position of the bird relative to the speaker’s head. Option A, across, is incorrect because it suggests that the bird flew from one side of the speaker’s head to the other. Option C, beside, is incorrect because it suggests that the bird flew next to the speaker’s head. Option D, behind, is incorrect because it suggests that the bird flew behind the speaker’s head.
The word “over” is used to describe the position of something that is above or on top of something else. In this case, the bird flew above the speaker’s head.