This is because, when you graph the trigonometric functions, which can be reresented as: a*(trig sign)*bx + K, the period of the function for sin, cos, csc, and sec are all 2pi divided by "b". However, the formula to find the function of tan or cot graphs is pi divided by b. And, since we don't see any values being multilied to the x in the choices, and we calculate the period for each of them, it concludes that tanx, has a period of pi
