c
Error always gets added so,
$\mathrm{g}=4 \pi^2\left(\frac{\mathrm{l}}{\mathrm{T}^2}\right) $
$\frac{\Delta \mathrm{g}}{\mathrm{g}}=\frac{\Delta 1}{\mathrm{~L}}+\frac{2 \Delta \mathrm{T}}{\mathrm{T}} $
$\frac{\Delta \mathrm{g}}{\mathrm{g}} \times 100=1+2(3) $
$\frac{\Delta \mathrm{g}}{\mathrm{g}} \times 100=7 $
Percentage error $=7 \%$