If the average monthly return and volatility of ASX $200$ is $20\%$ and $28\%$ respectively and the average monthly return on U$.$S$.$ treasury bills is $6\%.$ Calculate Sharpe ratio.

a$)1\mathrm{.}40\%$

b$)0\mathrm{.}50\%$

c$)0\mathrm{.}71\%$

d$)2\mathrm{.}00\%$