a b c d

a

$\text {Power factor Initially } cos\phi = \frac{R}{Z_i} = \frac{R}{\sqrt {R^2 + X_L^2}} = \frac{1}{\sqrt 10}$ $ \text{New Power Factor }cos\phi'= \frac{R}{Z_f} = \frac{R}{\sqrt {R^2 + (X_L-X_C)^2}} = \frac{1}{\sqrt 5}$ $\text{Ratio }= \sqrt 2$