An element with mass number $81$ contains $31.7\%$ more neutrons as compared to protons. Assign the atomic symbol.

(D) Let the number of protons in the element be $x$.
$\therefore$ Number of neutrons in the element
$= x + 31.7\% \text{ of } x$
$= x + 0.317x = 1.317x$
According to the question,the mass number is $81$.
$\therefore \text{Number of protons} + \text{Number of neutrons} = 81$
$x + 1.317x = 81$
$2.317x = 81$
$x = \frac{81}{2.317} \approx 34.95 \approx 35$
Since the atomic number is equal to the number of protons,the atomic number is $35$.
The element with atomic number $35$ is Bromine $(Br)$.
$\therefore$ The atomic symbol is $_{35}^{81}Br$.