Introduction:
In mathematics, the concept of closure under an operation refers to the property of a set that states that when two elements in the set are combined using the operation, the result is also an element of the set. In other words, if a set is closed under an operation, performing that operation on any two elements in the set will always yield another element within the set.

Closure under Addition:
Closure under addition means that when two numbers are added, the result should also be a number within the same set. For example, if we have a set of rational numbers, closure under addition would mean that when we add any two rational numbers, the sum will also be a rational number.

Explanation:
- Irrational numbers: Irrational numbers are numbers that cannot be expressed as a fraction or a ratio of two integers. For example, √2 and π are irrational numbers. When we add two irrational numbers, the result is not necessarily an irrational number. For instance, √2 + √2 = 2√2, which is still an irrational number. Therefore, irrational numbers are not closed under addition.
- Negative numbers: Negative numbers are integers less than zero. When we add two negative numbers, the sum is still a negative number. For example, -3 + (-4) = -7. Hence, negative numbers are closed under addition.
- Rational numbers: Rational numbers are numbers that can be expressed as a fraction or a ratio of two integers. For example, 1/2 and -3/4 are rational numbers. When we add two rational numbers, the result is always a rational number. For instance, 1/2 + (-3/4) = (-1/4), which is still a rational number. Therefore, rational numbers are closed under addition.

Conclusion:
Based on the explanation above, the correct answer is option 'c' - Rational numbers. Rational numbers are closed under addition, meaning that when two rational numbers are added, the result will always be another rational number.