Question

The light-gathering power of a 4-meter telescope is ________ than that of a 2-meter telescope.

Answer 1

The "meters" measurements refer to the diameter of the objective (the main big-moose lens or mirror).
But light collection depends on the AREA of the objective, and that means the SQUARE of the diameter.
So if you double the 'size' if the telescope, it gathers FOUR TIMES as much light as before.