A student measures the time period of a simple pendulum 4 times....

First, find the fourth reading.
The mean of $4$ readings is $2.00\text{ s}$, so their sum is $4 \times 2.00 = 8.00\text{ s}$.
The sum of the three given readings is $1.98 + 1.99 + 2.02 = 5.99\text{ s}$.
Therefore, the fourth reading is $8.00 - 5.99 = 2.01\text{ s}$.

Next, calculate the absolute errors for each reading:
$\Delta T_1 = |1.98 - 2.00| = 0.02\text{ s}$
$\Delta T_2 = |1.99 - 2.00| = 0.01\text{ s}$
$\Delta T_3 = |2.02 - 2.00| = 0.02\text{ s}$
$\Delta T_4 = |2.01 - 2.00| = 0.01\text{ s}$

Now, find the mean absolute error:
$\Delta T_m = \dfrac{0.02 + 0.01 + 0.02 + 0.01}{4} = \dfrac{0.06}{4} = 0.015\text{ s}$

Finally, calculate the percentage relative error:
$\%\text{ error} = \left(\dfrac{\Delta T_m}{T_m}\right) \times 100$
$\%\text{ error} = \left(\dfrac{0.015}{2.00}\right) \times 100 = 0.75\%$

Answer: $0.75\%$