That's a lot of questions in one post.
You are correct - the answer to number 1 is (c). You can see from the graph that it does one revolution in 4 seconds, so it will do 5 revolutions in 20 seconds.
All of the rest of your answers have to do with your equation y = a cos[b(t - c)] + d: finding or interpreting values for a, b, c, and/or d. So what do these parameters represent?
a is the amplitude - one half the difference between the maximum and minimum.
d is the average value - (max + min)/2
b has to do with how fast y goes up and down - it is divided by the length of a whole cycle
c is used to adjust the graph to the right or left - it is one of the values of t where y is at its maximum
I hope this helps you on your way. If you are having problems with one of the questions in particular, please post again.