For a standard normal distribution, which of the following expressions must always be equal to 1?
a) p(z≤-a)-p(-a≤z≤a)+p(z≥a)
b) p(z≤-a)-p(-a≤z≤a)+p(z≥a)
c) p(z≤-a)+p(-a≤z≤a)-p(z≥a)
d) p(z≤-a)+p(-a≤z≤a)+p(z≥a)

P(z ≤ -a) + P(-a ≤ z ≤ a) + P(z ≥ a) = 1 - P(z ≤ a) + [P(z ≤ a) - P(z ≤ -a)] + 1 - P(z ≤ a) = 2 - 2P(z ≤ a) + P(z ≤ a) - [1 - P(z ≤ a)] = 2 - P(z ≤ a) - 1 + P(z ≤ a) = 1

Therefore, option D is the correct answer.