1. Write the given nuclear reaction

We need to identify the particle $R$.

2. Use conservation of mass number

In any nuclear reaction, the total mass number must be conserved.

• Left side mass number: $236$
• Right side mass number: $141 + 92 + 3A_R$

So,

So each emitted particle has mass number $1$.

3. Use conservation of atomic number

The total atomic number must also be conserved.

• Left side atomic number: $92$
• Right side atomic number: $56 + 36 + 3Z_R$

Thus,

So each emitted particle has atomic number $0$.

4. Identify the particle

A particle with:

• mass number $1$
• atomic number $0$

is a neutron.

Thus,

$R = {}_0n^1$

5. Check options

• A: Proton $\rightarrow A=1, Z=1$ ❌
• B: Neutron $\rightarrow A=1, Z=0$ ✅
• C: Electron $\rightarrow A=0, Z=-1$ ❌
• D: $\gamma$-radiations $\rightarrow A=0, Z=0$ ❌

Therefore, the correct option is B.