$(A)$ $u=0.8 v$ and $f_5=f_0$
$(B)$ $u=0.8 v$ and $f_5=2 f_0$
$(C)$ $u=0.8 v$ and $f_5=0.5 f_0$
$(D)$ $u=0.5 v$ and $f_5=1.5 f_0$
$(A)$ $f=f_0\left(\frac{v}{v-0.8 v}\right)=5 f_0$
$(B)$ $f =2 f _0\left(\frac{ v }{ v -0.8 v }\right)=10 f _0$
$(C)$ $f =0.5 f _0\left(\frac{ v }{ v -0.8 v }\right)=2.5 f _0$
$(D)$ $f =1.5 f _0\left(\frac{ v }{ v -0.5 v }\right)=3 f _0$
All odd harmonics are available in closed pipe therefore correct Ans $(A,D)$
$(A)$ With a node at $O$, the minimum frequency of vibration of the composite string is $v_0$
$(B)$ With an antinode at $O$, the minimum frequency of vibration of the composite string is $2 v_0$
$(C)$ When the composite string vibrates at the minimum frequency with a node at $O$, it has $6$ nodes, including the end nodes
$(D)$ No vibrational mode with an antinode at $O$ is possible for the composite string
(image)
[$A$] The time $\mathrm{T}_{A 0}=\mathrm{T}_{\mathrm{OA}}$
[$B$] The velocities of the two pulses (Pulse $1$ and Pulse $2$) are the same at the midpoint of rope.
[$C$] The wavelength of Pulse $1$ becomes longer when it reaches point $A$.
[$D$] The velocity of any pulse along the rope is independent of its frequency and wavelength.