For a standard normal distribution, which of the following expressions must always be equal to 1?

For a standard normal distribution, the expression that is always equal to 1 is P(z≤-a)+P(-a≤z≤a)+P(Z≥a). This expression represents all of the possible values in a curve, or in other words, the total area of a curve. According to standard normal distribution, the total area of a curve is always equal to 1.