C seems to make the most sense, logarithmic and exponential functions are not gradual, and a step function just wouldn't fit in my eyes.
Answer 1: 
Answer 2: will
Answer 3: $10
Step-by-step explanation:
Equation: 
Registration fee is one time, so you just add to it.
You have to pay each time you go to a session, so x will be the amount of sessions, 35 will be the price of each session.
You can plug in the 8 to the equation, then solve for y:

They will be able to pay for it because: 
They will have $10 left because:

340 is how much on the card and 330 is how much the 8 lessons (in total) cost.
Do you have a picture of the question??
the correct answer is C to this question
Step-by-step explanation:
The answer for third is D, you have multiple choice in fourth which I can't see at all...