Since, Seetha is $7^{th}$ from the left and Leena is $9^{th}$ from the right, when Seetha replace Leena, the position of Seetha from the right = $9.$ As per question, Seetha’s position from the left after interchanging $=11.$
Hence, the no. of girls in the row
$$\begin{align}&=\text{(Seetha’s position from the left + Seetha’s position from the right)-1}\\&=(9+11) -1\\&= 20-1\\&= 19\end{align}$$
So, answer is None of the above.