All the answers are correct.
The shape that is generated is a Cone.
A triangle, when rotated about one of it's side will generate a solid in a form of a Cone. The cone could be a hollow one or a solid filled one, depending on the properties of the triangle being rotated.
Slope of a line passing through these two points is 
.
Hope this helps.
The period is equal to 2pi/n, where n is the coefficient of t. In this case pi/2. Therefore the period in this example is 4. Frequency is equal to 1/period. Hence the frequency for this problem is 1/4
To write the equation you start with the 125 because it is monthly and it doesn't say how often you pay the 75 dollars so the equation would be c(f)=125x+75
