If the sum of two unit vectors is a unit vector, then the angle between them is
Correct: D
Let the two unit vectors be a and b. We have |a| = |b| = 1 and |a + b| = 1. Squaring |a + b| = 1, we get (a + b) · (a + b) = 1. Expanding gives a · a + 2a · b + b · b = 1. Since |a| = |b| = 1, a · a = b · b = 1. Then 2 + 2a · b = 1, so a · b = -1/2. Thus, the angle between a and b is given by cos θ = -1/2, so θ = 120°.