The mean value of time period, $T =\frac{\sum T _{ i }}{5}=\frac{2.78}{5}=0.56 s$

Magnitude of absolute error in each observation,

$\left|\Delta T _1\right|=|0.56-0.52|=|0.04| s$

Similarly, $\left|\Delta T _2\right|=|0.0| s \left|\Delta T _3\right|=|0.01| s \left|\Delta T _4\right|=|0.02| s \left|\Delta T _5\right|=|0.03| s$

Mean absolute error in time period,

$\Delta T _{ m }=\frac{0.04+0.00+0.01+0.02+0.03}{5}=0.02 s$

$\therefore$ Error in $T , \frac{\Delta T _{ m }}{ T } \times 100=\frac{0.02}{0.56} \times 100=3.57 \%$

Error in the measurement of $r : \frac{\Delta r }{ r } \times 100=\frac{1}{10} \times 100=10 \%$

From the equation given, we get: $g =\frac{28 \pi^2( R - r )}{5 T^2}$

$\therefore$ Error in the measurement of $g$ :

$\frac{\Delta g }{ g } \times 100=\frac{\Delta R +\Delta r }{( R - r )} \times 100+2 \frac{\Delta T _{ m }}{ T } \times 100$

$\Rightarrow \frac{\Delta g }{ g } \times 100=\frac{1+1}{(60-10)} \times 100+2(3.57) \%=11.14 \%$

Thus options $A, B$ and $D$ are correct.