The ac schematic of an $NMOS$ common-source stage is shown in the figure below, where part of the biasing circuits has been omitted for simplicity. For the $n$-channel  $MOSFET\: M,$ the transconductance $g_{m} = 1\; mA/V,$ and body effect and channel length modulation effect are to be neglected. The lower cutoff frequency in $Hz$ of the circuit is approximately at
1. $8$
2. $32$
3. $50$
4. $200$