The lateral surface area of a cylinder is just the long side connecting the bases, whereas the total surface area includes both the lateral surface area and area of the two circular bases.

(Imagine a rectangle with the circumfrence of the base as one side and the height as the other being folded around)

(Plus the area of two circles)
The literal difference between surface areas would be the two bases 2πr² if you need that.
You need to order the data, so
11, 12, 13, 14, 14, 15, 17, 17, 18, 18, 18, 20
Then find the middle of the data. Because there are 12 data points, the median will lie between the two 6th placed numbers
The two numbers are 15 and 17, so work out the mean of them. (15+17)/2 = 16
So the answer is B. 16
2 x 10 = 20
Check: 20 / 2 = 10
n = 20