A gas expands isothermally and reversibly. The work done by the gas is

Expansion of gas means work is done by the system on surroundings and the value of the work done is negative.

Work done in reversible process is $\mathrm{W}=-2.303\mathrm{nRT}\log V_2/{\mathrm{V}}_1$

The work done by the system in an isothermally and reversibly process is always greater than irreversibly process.

Hence the correct option is (B).