Well, in this case you can draw a Venna diagram to help you.

Let's say they are 100 members in total.
30 members use Pool and Sauna (1)
40% of the members who use the Pool do not use the Sauna, they only use the pool. It also tells you that 60% of the members who use the pool also use the sauna.(2)

Joining (1) and (2) you know that the 30 members represent 60% of the members who use the pool.
If X is the number of members who use the pool:
30=60/100X
X=50

And 50 members out of 100 is ... 50%

i would go with D . 50%

the question stem tell us that 30% of people use pool and sauna and 40% per cent of the people who use the pool don't use the sauna.

whic means 30% of the people who use both the pool and sauna are 60% of the pool users.

therefore 0.3 = 0.6X.

therefore X = 50%.

Hope it helps. do let me know if u have any doubts..

Thanks,