Final answer:
The smallest positive number for which cosine x equals 0 is π/2, as this is the point on the unit circle where the x-coordinate is zero.
Step-by-step explanation:
The smallest positive number x for which the cosine of x equals 0 is π/2. This can be determined from the basic trigonometric properties of a unit circle, where the cosine of an angle represents the x-coordinate of a point on the unit circle. The cosine of π/2 is 0 because at this angle, the corresponding point on the unit circle lies on the y-axis, making the x-coordinate zero.
Therefore, the correct answer to the question is: a) π/2.