Explanation:

- Order of a differential equation: It is defined as the highest derivative present in the equation.

- Option 1: Rational number

- Rational numbers can be fractions or whole numbers.

- The order of a differential equation cannot be fractional.

- Option 2: Positive integer

- The order of a differential equation must be a positive integer.

- It refers to the highest order of derivative, which is counted as first-order, second-order, etc.

- This is the correct option.

- Option 3: Negative integer

- Negative integers are not used for the order of derivatives.

- Option 4: Whole number

- Whole numbers include zero, but zero is not a valid order for a differential equation, as it has at least a first-order term.