How to calculate the area of a rectangle?

I - The area of a rectangle is obtained by multiplying the length of its sides.
II - The area of a rectangle is the portion of the plane it covers.
III - The unit of measurement used to measure surfaces is the square whose side is of length 1.
a) Only I is correct.
b) Only II is correct.
c) Only III is correct.
d) I and II are correct.
e) I, II, and III are correct.