( E T )_ V is the mathematical expression for

Answer: Option D

By the definition of $$C_V$$ we can write $$C_V=\underset{\delta T\to 0}{\lim }{\left(\frac{\delta Q}{\delta T}\right)}_V.$$
Since, under constant volume process the expansion work is zero, and by first law of thermodynamics
$$\begin{array}{rl}& \delta Q=dU\\ & \Rightarrow {\left(\frac{\partial U}{\partial T}\right)}_V=C_V.\end{array}$$