A line AB in three-dimensional space makes angles 45° and 120° with the positive x-axis and the positive y-axis respectively. If AB makes an acute angle θ with the positive z-axis, then θ equals 45°, 75°, 60°, 30°

The angle made by line AB with x-axis is 45°
The angle made by line AB with y-axis is 120°
Let the angle made by line AB with z-axis be θ
We have cos²(45°) + cos²(120°) + cos²(θ) = 1
⇒ cos²(θ) = 1 - ½ - ¼ = ¼
Given that θ is acute
So we get cos(θ) = ½ ⇒ θ = 60°
Therefore option B is correct