Step 1: Understanding isothermal compressibility.
Isothermal compressibility describes how the volume of a substance changes with pressure at constant temperature. It is defined as:
\[
\beta_T = -\frac{1}{V} \left( \frac{\partial V}{\partial P} \right)_T
\]
where \( \beta_T \) is the isothermal compressibility, \( V \) is the volume, \( P \) is the pressure, and \( T \) is the temperature.
Step 2: Identifying the correct option.
The correct form of isothermal compressibility is \( \beta_T = -\frac{1}{V} \left( \frac{\partial V}{\partial P} \right)_T \), which matches option (C).