How can u determine if a number is a rational number

If the number can be written as a ration (fraction) in  form, it is rational.

All integers can be written in that form :  for example - 7 =  and so on.
Repeaters such as .333... can also be written in fraction form.  It is another lesson to explain how, but you can prove it to yourself by dividing 1 by 3
You will get repeating 3's so that means repeating three are the same as the rational number 1/3

What you cannot write as a ratio (fraction form) is a number like √5 because the answer is a number that does not repeat and keeps going on forever.  There is no 
exact number that when multiplied by itself gives you 5.  ∨5 is irrational.

C. expression because any expression or equation may contain more than one variable