b
(b)
Length of pendulum = Length of thread + Radius of bob
$=63.2+\frac{2.256}{2}=63.2+1128$
$=64.328 \,cm$
But now the student must apply rule for taking significant digits in a measurement.
In addition or subtraction,
Number of digits after decimal in result
= Least number of digits after decimal in quantities added
So, length of pendulum $=64.3 \,cm$.