Which change will triple the lateral surface area of a cone?

triple the radius and the slant height.

increase the height so that the slant height triples.

triple the height but do not change the radius.

increase the height and the slant height so that the radius triples.

The answer for this would be the second answer choice
This is because when the slant height is tripled, the overall lateral surface area is tripled
The best way to explain this would be through an example. 

Say that the lateral height is 2 and the radius is 1. The lateral surface area is 6.28

Now, if I make the lateral height 6, which is triple the original lateral height, the lateral surface area becomes 18.84, which is triple the original surface area

Therefore, the answer is choice 2

Hope this helped!! :D