It is given that, four persons $P, Q, R$, and $S$ are to be seated in a row. $R$ should not be seated at the second position from the left end of the row.
Now, we can make the possible arrangements.
- From the left end, the second position can be filled in $3$ ways.
- From the left end, the first position can be filled in $3$ ways.
- From the right end, the second position can be filled in $2$ ways.
- From the right end, the first position can be filled in $1$ way.
$$\boxed{\underset{1}{3}\quad \underset{2} 3 \quad\underset{3} 2 \quad \underset{4}1}$$
$\therefore$ The number of distinct seating arrangements possible $ = 3 \times 3 \times 2 \times 1 = 18.$
So, the correct answer is $(C).$