Calculating LL(1) grammar
It is evident that $B$ can be followed by $a$  &mdash; in other words, that $a\in \text{FOLLOW}(B)$ &mdash; as shown by the partial derivation

$$\begin{align}S&amp;\to A B C&amp;(S\to ABC)\\
&amp;\to  AaBC &amp;(A\to Aa)\\&amp;\to aBaBC&amp;(A\to aB) \\
\end{align}$$

So the answer provided does not correspond to this grammar.