Using a metre scale, the length of the string of a simple pendulum...

The effective length of the pendulum is the sum of the length of the string and the radius of the bob plus the length of the hook.

$L = l_1 + l_2$

Given $l_1 = 90.0 \text{ cm}$ and $l_2 = 2.13 \text{ cm}$.

$L = 90.0 + 2.13 = 92.13 \text{ cm}$

According to the rules of significant figures for addition, the final result must retain as many decimal places as there are in the number with the least decimal places. Since $90.0$ has only one decimal place, the result must be rounded off to one decimal place.

Therefore, the effective length is $92.1 \text{ cm}$.