The small-signal resistance $(i.e., dV_{B}/dI_{D})$ in $k\Omega$ offered by the $n$-channel $MOSFET\: M$ shown in the figure below, at a bias point of $V_{B} = 2\: V$ is (device data for $M:$ device transconductance parameter $k_{N} = \mu_{n} C'_{ox} (W/L) = 40\: \mu A/V^{2},$threshold voltage $V_{TN} = 1\: V,$ and neglect body effect and channel length modulation effects)
- $12.5$
- $25$
- $50$
- $100$