Given below are two statements : One is labelled as Assertion (A) and the other is labelled as Reason (R). 

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

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

In the light of the above statements, choose the correct answer from the options given below.

Assertion (A): Enthalpy of neutralisation of strong monobasic acid with strong monoacidic base is always $$-57$$ kJ/mol.

This is true. For strong acid-strong base neutralisation, the net reaction is simply $$H^+ + OH^- \rightarrow H_2O$$, and the enthalpy is $$-57.1$$ kJ/mol (approximately $$-57$$ kJ/mol).

Reason (R): Enthalpy of neutralisation is the heat liberated when 1 mol $$H^+$$ combines with 1 mol $$OH^-$$ to form 1 mol water.

This is true and is the definition of enthalpy of neutralisation.

Since both are true and (R) correctly explains why (A) is always $$-57$$ kJ/mol (because for strong acid and strong base, the only reaction is $$H^+ + OH^- \rightarrow H_2O$$, there's no extra energy for dissociation), (R) is the correct explanation of (A).

The correct answer is Option (3): Both (A) and (R) are true and (R) is the correct explanation of (A).