A sequence is a function whose domain is the set of natural numbers or a subset of the natural numbers. We usually use the symbol an to represent a sequence, where n is a natural number and an is the value of the function on n.

**Explanation:**

A sequence is a function that maps the natural numbers (also known as positive integers) to a set of numbers. In other words, a sequence is a list of numbers that are arranged in a specific order.

Let's break down the given options and understand why the correct answer is option 'A':

a) **Natural Numbers:**
The natural numbers are the set of positive integers starting from 1. It includes numbers like 1, 2, 3, 4, 5, and so on. A sequence can be defined for these numbers, where each number represents a position in the sequence.

b) **Real Numbers:**
The real numbers include both positive and negative numbers, along with fractions and decimals. A sequence can certainly be defined for real numbers as well, but the question specifically mentions "natural numbers" as the domain, so this option is incorrect.

c) **Whole Numbers:**
The whole numbers are the set of positive integers along with zero. It includes numbers like 0, 1, 2, 3, 4, 5, and so on. While a sequence can also be defined for whole numbers, the question specifically asks for the domain to be "natural numbers," so this option is incorrect.

d) **Integers:**
The integers include both positive and negative whole numbers, along with zero. It includes numbers like -3, -2, -1, 0, 1, 2, 3, and so on. While a sequence can also be defined for integers, the question specifically asks for the domain to be "natural numbers," so this option is incorrect.

Therefore, the correct answer is option 'A' - Natural Numbers, because a sequence is a function that maps the natural numbers to a set of numbers.

Answer should be (B).