A nuclear transformation is denoted by $X(n,\alpha ){}_3^7\text{Li}$. Which of the following is the nucleus of element $X$?

The notation $X(n,\alpha ){}_3^7\text{Li}$ represents the reaction:
$${}_Z^{A}X+{}_0^{1}n\to {}_3^7\text{Li}+{}_2^4\text{He}$$
Conservation of Mass Number ($A$):
$$A+1=7+4⟹A=10$$
Conservation of Atomic Number ($Z$):
$$Z+0=3+2⟹Z=5$$
The element with $Z=5$ is Boron ($\text{B}$). Thus, the nucleus is ${}^{10}{\text{B}}_5$.