I am also having the same doubt for few weeks. I asked it over here: https://gateoverflow.in/165929/doubt-in-pipelining-questions
Read through all the comments in both the GATE questions. Especially by Arjun sir.
In one case, we have considered multistage buffers in which the result of a stage is saved in a queue. So further instructions can use the stage. But in other, we have considered that a stage cannot be used until it's buffers are free.
What I concluded is that it's a matter of matching options. In case of numerical question I will assume interstage buffers to be queue.