Assertion (A): The bond angle in trimethylamine is $108°$ 

Reason (R): In trimethylamine nitrogen is ${\mathrm{sp}}^3$ hybridised.

For triethylamine,

Total number of hybrid orbitals $=$ number of sigma bonds $+$ number of lone pairs of electrons

$$\begin{gathered} =(3+1) \\ =4 \end{gathered}$$

So, hybridisation is ${\mathrm{sp}}^3$.

$\Rightarrow$ Shape $=$ trigonal pyramidal

$\Rightarrow$ Bond angle $=108°$

Thus, Both assertion and reason are true and the reason is the correct explanation of assertion Hence, option (A) is correct.