Which of the following is not an integer?
A. $ 2 \div 7 $
B. $ 2 - 0 $
C. $ - 5 \times 9 $
D. $ - 13 + 17 $

Hint: Integers can be defined as the number which can be written without the fractional component. It can be positive or the negative number. Here we will take all the given expressions in the options and simplify to check whether they are integers or not.

Complete step-by-step answer:
A. $ 2 \div 7 $
It can be re-written as –
 $ = \dfrac{2}{7} $
A rational number is the number which can be expressed as the ratio of two numbers or which can be expressed as the p/q form or as the quotient or the fraction with non-zero denominator.
So, the given number is the rational number and not the integer.

B. $ 2 - 0 $
Simplify the above expression-
 $ = 2 $
Hence, the given number is an integer.

C. $ - 5 \times 9 $
Simplify the above expression –
 $ = ( - 45) $
Hence, the given number is an integer.

D. $ - 13 + 17 $
Simplify the above expression. Remember when you add one negative and one positive number, you have to subtract and give the sign of a bigger digit.
 $ = (4) $
Hence, the given number is an integer.
So, the correct answer is “Option A”.

Note: Also, refer to the terminology for natural numbers, whole numbers, rational and irrational numbers and know the difference between them. The set of the real numbers is made by combining the set of all the rational numbers and all the set of irrational numbers. The real numbers include natural numbers, whole numbers, integers, and the rational numbers including all the fractions, decimals and repeating numbers and irrational numbers.