E(X) of a random variable X, the theoretical mean of X. It is not based on the sample data but on the distribution of it. So, mean expectation value is a:

Parameter

The correct answer is Option (3) → Parameter

$E(X)\; \text{is based on the probability distribution, not on sample data}$

$\text{Hence it describes a characteristic of the population}$

$\Rightarrow \text{Parameter}$

The mean expectation value is a parameter.