b
Here, \(T=2 \pi \sqrt{\frac{L}{g}}\) or \(T^{2}=4 \pi^{2}(L / g)\)

So, \(g=\frac{4 \pi^{2} L}{T^{2}}\)

Thus, \(\frac{\Delta g}{g}=\frac{\Delta L}{L}+2 \frac{\Delta T}{T}\)

% error in \(g=\frac{\Delta g}{g} \times 100\)

\(=\left(\frac{\Delta L}{L}+2 \frac{\Delta T}{T}\right) \times 100\)

\(=\left(\frac{(1 / 10)}{20}+2 \times \frac{1}{90}\right) \times 100=2.72 \%\)