The process capability measures $C_p$ and $C_{pk}$ differ because:

A$)C_p$ values for a given process will always be greater than or equal to $C_{pk}$ values.

B$)C_p$ values above $1$ indicate a capable process, while $C_{pk}$ values above $2$ indicate a capable process.

C$)$ They do not differ: both are identical.

D$)$ only one ensures the process mean is centered within the limits$.$

E$)$ Both only one ensures the process mean is centered within the limits and $C_p$ values for a given process will always be greater than or equal to $C_{pk}$ values are correct.