If all of the inexperienced plumbers are assigned to the team, it means that we must leave out one of the experienced plumbers - so we pick all four of inexperienced - R S T V and four of the experienced plumbers, resulting in four E/I pairs:
E E E E I I I I
The question asks us which team S must be assigned to if neither R nor T nor V is assigned to the team with G.
There are two possible scenarios for this question - G is entirely left out or G is assigned to one of the teams. Let's consider each of them:
(1) G is assigned
If G is assigned to one of the teams, but neither R nor T nor V can be assigned with him, then the only remaining inexperienced plumber that could be assigned with him is S.
G S
So in a scenario where G is assigned to one of the team, S must be assigned to the same team.
(2) G is not assigned to any team.
If G is not assigned to any team, then we have to pair F J K M with R S T V. Per the rules, F cannot be assigned with R or V or T as T must be assigned with K if G is a part of the assignment, hence the only inexperienced plumbers left for F is S. Thus, we can conclude that if G is not assigned to any team, S must be assigned to a team with F .
The correct answer choice (A) reflects both of these scenarios.
(B) is incorrect because if S is assigned to a team with M, and G cannot be assigned with R or T or V, there are no inexperienced plumbers left to pair up with G in this scenario. Hence, if G is picked, S must be assigned to the team with him.
Let me know if this makes sense and if you have any further questions.