What would happen if water from tank a was added to tank b?
a. the salinity in tank a would be less
b. the salinity in tank a would be higher
c. the salinity in tank b would be less
d. the salinity in tank b would be higher

When you take water from tank A then tank A salinity would not be less or higher so option A and B are out. The answer is depends on the relative salinity of tank A to tank B.
Assuming the tank A salinity is higher than B, then the tank B salinity will also become higher.
 d.the salinity in Tank B would be higher