Experts answer it asap.....

Dear student,

${}_Z{}^{A}X$, where A is the atomic mass & Z is the atomic number of the element.
Number of electrons=15,
Number of neutrons=16,
Atomic number = number of protons= number of electrons=15 
The atomic mass (number of electrons + Number of neutrons) in this case is 15+16=31,
Therefore the atom would be represented as: ${}_{15}{}^{31}X$

​Therefore, option A is correct.

Regards