$A \rightarrow(p, t):$ It is a closed organ pipe hence in case of fundamental vibrations length of the tube must be $\left(\frac{1}{4}\right)^{\text {th }}$ of the wavelength (half loop).

$\frac{\lambda_t}{4}=L \Rightarrow \lambda_t=4 L$

$B \rightarrow(p, s)$ : It is open organ pipe hence in case of fundamental vibrations length of the pipe must be half of the wavelength (one loop).

$\frac{\lambda_t}{2}=L \Rightarrow \lambda_t=2 L$

$C \rightarrow(q, s)$ : It is a case of string of length $L$ fixed at both ends.

In case of fundamental vibrations length of the pipe must be half of the wavelength (one loop).

$\frac{\lambda_t}{2}=L \Rightarrow \lambda_t=2 L$

$D \rightarrow(q, r):$ In this case mid-point is fixed hence half-length must form one loop.

$\frac{\lambda_t}{2}=\frac{L}{2} \Rightarrow \lambda_t=L$