1. Interpret the nuclear reaction notation

   The notation $X(n,\alpha)\,{}^{7}_{3}\text{Li}$ means that nucleus $X$ is bombarded by a neutron $n$, and an alpha particle $\alpha$ is emitted, producing ${}^{7}_{3}\text{Li}$.

   So the reaction is: $X + {}^{1}_{0}n \rightarrow {}^{4}_{2}\alpha + {}^{7}_{3}\text{Li}$

2. Apply conservation of mass number

   Let $X = {}^{A}_{Z}X$.

   Mass numbers must balance: $A + 1 = 4 + 7 = 11$ $A = 10$

3. Apply conservation of atomic number

   Atomic numbers must balance: $Z + 0 = 2 + 3 = 5$ $Z = 5$

4. Identify the nucleus

   A nucleus with:

   • mass number $A = 10$
   • atomic number $Z = 5$

   is $^{10}_{5}\text{B}$

5. Check options

   • A: ${}^{10}_{5}B$ ✅
   • B: ${}^{12}_{6}C$ ❌
   • C: ${}^{11}_{4}Be$ ❌
   • D: ${}^{9}_{5}B$ ❌

Therefore, the correct answer is A.