The correct option is B −848.2 cal
We know, work done for an isothermal process is given as:
W=−Pext(V2−V1)
where Pext= external pressure = 1 atm
V2= final volume = 50 L
V1 = initial volume = 15 L
W=−1×35=−35 L atm
and 1 L atm=101.3 J
So, W=−35×101.3 J=−3545.5 J
We know 1 cal=4.18 J
W=−3545.54.18 cal
so, W=−848.2 cal