Right answer is (b) Vs/2
The best I can explain: As only 2 SCRs are conducting at a time, it is 120 mode.
When T1 and T2 are conducting, R phase is connected to the positive bus through the T1 SCR and B phase is connected to the negative bus through the T2 SCR.
Considering that the load is a balanced R load.
VRN = Vs/2
VYN = -Vs/2.