This question is really simple and the answer is letter a and I'll explain you why.
The purpose of an equation is to try to reproduce the same pattern we need. So in our system always, we enter with a number of cups of water(x) we get a number of tea bags(y), so we need to find which equation fit our pattern. We can do that by taking each of the equations and trying to substitute x with our number of cups of water (2,3,6,7,9 or 10), and this equation needs to give us the corespondent number of tea bags (5,7,13,15,19 or 21). When we try with the first equation in the letter A as in the pic above. we can see we fit what we need. So when we enter x = 2 we get y = 5; when we enter x = 3 we get y = 7 and if we try this equation with all possible values of x we always get the corresponding y value as presented in the table.