On interval 0 ≤ x < 2π, where are the x-intercepts of y = cos(2x)?

The answer is choice D because we plug in something like x = pi/4 and it leads to

y = cos(2x)

y = cos(2*pi/4)

y = cos(pi/2)

y = 0

Use the unit circle to see why cos(pi/2) = 0.

Since we get y = 0 as a result, this shows x = pi/4 is one of the x intercepts.

You'll also get y = 0 if you plug in x = 3pi/4, x = 5pi/4, and x = 7pi/4. All of these values are on the interval