How many significant figures are in 7.0

Trailing zeros to the right of the decimal ARE significant.
So, in 7.0 , there are 2 significant figures.

Another example: There are FOUR significant figures in 92.00

There are two significant figures in 7.0 because 7.0 is the greater than 1. If there is a decimal number that is less than 1, all figures are significant except the leading zeros after its decimal point and for a decimal number that has one or more zeros at the end, the very last zeros count as significant. so, 7.0 is the decimal number, we count both figure 7 and 0 and a significant figures.

C) electrons encircling a nucleus with neutrons and protons

Depends on the speed of the car.