Correct option is A
An operator is a symbol that tells you to do something to whatever follows the symbol.
x component of the linear momentum can be represented in quantum mechanics by an operator of the form

The Expectation Value
If the system is in a state described by the wave function

and the value of the observable a is measured once on each of many identically prepared systems, the average value (also called the expectation value) of all of these measurements is given by






