Assertion (A): When acetamide reacts with $\mathrm{NaOH}$ and ${\mathrm{Br}}_2$, methyl amine is formed. 

Reason (R): The reaction occurs through intermediate formation of isocyanate.

The reaction between acetamide and $\mathrm{NaOH}/{\mathrm{Br}}_2$,

${\mathrm{RCONH}}_2+4\mathrm{NaOH}+{\mathrm{Br}}_2\to {\mathrm{RNH}}_2+{\mathrm{Na}}_2{\mathrm{CO}}_3+2\mathrm{NaBr}+2{\mathrm{H}}_2\mathrm{O}$

Mechanism:                         

It is clear from the mechanism that the reaction proceeds through the formation of isocyanate as an intermediate.

Thus, both assertion and reason are true and the reason is the correct explanation of the assertion

Hence, option (A) is correct.