For a constant volume process

Answer: Option B

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