Analyze the Assertion (A)

Assertion: Enthalpy of neutralisation of strong monobasic acid with strong monoacidic base is always $-57\,\text{kJ mol}^{-1}$.

For a strong acid and a strong base, both are almost completely ionised in aqueous solution: $\mathrm{HCl \rightarrow H^+ + Cl^-}$ $\mathrm{NaOH \rightarrow Na^+ + OH^-}$

So the actual net ionic reaction is: $\mathrm{H^+ (aq) + OH^- (aq) \rightarrow H_2O(l)}$

The enthalpy change for this reaction is nearly constant and is approximately: $\Delta H_{\text{neut}} \approx -57\,\text{kJ mol}^{-1}$

Hence, Assertion (A) is true.

Analyze the Reason (R)

Reason: Enthalpy of neutralisation is the amount of heat liberated when one mole of $\mathrm{H^+}$ ions furnished by acid combine with one mole of $\mathrm{OH^-}$ ions furnished by base to form one mole of water.

This is the standard definition of enthalpy of neutralisation.

Therefore, Reason (R) is true.

Check whether (R) explains (A)

In the case of strong acid and strong base, the acid and base are fully dissociated. So neutralisation always reduces to the same ionic reaction: $\mathrm{H^+ + OH^- \rightarrow H_2O}$

Since the same process occurs in every such case, the enthalpy of neutralisation remains the same, about: $-57\,\text{kJ mol}^{-1}$

Thus, (R) correctly explains (A).

Correct option

Therefore, the correct answer is: B: Both (A) and (R) are true and (R) is the correct explanation of (A).