The electrical force between charges is strongest when the charges are
a) close together. b) far apart. c) the electric force is constant everywhere.

The electrical force is a force that is inversely proportional to the square of the distance. This can be proved by Coulomb's Law, which states:  

"The electrostatic force between two point charges and is proportional to the product of the charges and inversely proportional to the square of the distance that separates them, and has the direction of the line that joins them"  

Mathematically this law is written as:  

Where is a proportionality constant.

Therefore, if the charges are close together, this means the distance between them is minimal and the electrical force is increased.

d. air pressure decreases as altitude increases.