A student uses a simple pendulum of exactly 1 ~m length to...

T=\frac{40 \mathrm{~s}}{20}=2 \mathrm{~s}$. Further, t=n T=20 T or$\Delta t=20 \Delta T\therefore \quad \frac{\Delta t}{t}=\frac{\Delta T}{T} or \Delta T=\frac{T}{t} \cdot \Delta t=\left(\frac{2}{40}\right)(1)=0.05 \mathrm{~s}$Further,$T=2 \pi \sqrt{\frac{l}{g}}$or$\quad T \propto g^{-1 / 2}$\therefore$\frac{\Delta T}{T} \times$100=-$\frac{1}{2} \times \frac{\Delta g}{g} \times$100 or$\%$error in determination of$g$is$$\begin{aligned}\frac{\Delta g}{g} \times100 & =-200 \times \frac{\Delta T}{T}\\ & =-\frac{200 \times 0.05}{2}=-5 \% \end{aligned}$$\therefore$ correct options are (a) and (c).