In a real exponential function of the form \( f(x) = a^x \), the base \( a \) must be a positive real number and not equal to 1.
Let's evaluate the options you provided:
- -2: This cannot be the base of a real exponential function because a negative base is not valid for the real exponential function.
- π: This is a positive real number and can be used as the base.
- 1.5: This is also a positive real number and can be used as the base.
- 3/4: This is a positive real number (less than 1) and can be used as the base.
Therefore, the only option that cannot be the base of a real exponential function is -2.