The activity will be

$A=\lambda N$

Now,

$\lambda =\frac{\ln \left(2\right)}{t_{{\displaystyle \frac{1}{2}}}}$ and $N=\frac{1.5\times {10}^{-3}}{198}\times 6\times {10}^{23}$

Therefore, activity in $\mathrm{Ci}$,

$A=\frac{\ln 2}{t_{{\displaystyle \frac{1}{2}}}}\times \frac{1.5\times {10}^{-3}}{198}\times \frac{6\times {10}^{23}}{3.7\times {10}^{-10}}=\frac{\ln 2}{2.7\times 24\times 60\times 60}\times \frac{1.5\times {10}^{-3}}{198}\times \frac{6\times {10}^{23}}{3.7\times {10}^{-10}}=366 \mathrm{Ci}\approx 357 \mathrm{Ci}$