Explanation

This tests the bounds of squared trigonometric functions; the expression is cos²x + cos²y - cos²z (the transcription renders cos² with a superscript marker). Since cos²x ≥ 0 and cos²y ≥ 0 while -cos²z ≥ -1, the minimum is 0 + 0 - 1 = -1, attained at x = y = 90°, z = 0°. Option (b) 0 is the trap from forgetting the subtracted term can pull the total below zero. Hence option (a).