The index of the first element in an array is 0.

An array is a data structure that stores a collection of elements of the same type in contiguous memory locations. Each element in an array is accessed using its index, which represents its position within the array.

Understanding array indexing:
In most programming languages, array indexing starts from 0. This means that the first element in an array is accessed using an index of 0, the second element with an index of 1, and so on. The index value represents the offset from the beginning of the array.

Example:
Consider an array of integers named "numbers" with the following elements:
numbers = [5, 10, 15, 20]

To access the first element (5) in the array, you would use the index 0:
numbers[0] => 5

To access the second element (10), you would use the index 1:
numbers[1] => 10

Reasoning behind the index starting from 0:
The choice of starting array indexing from 0 is based on a few factors:

1. Simplicity: Starting indexing from 0 simplifies the computation of the memory address of each element in the array. It allows for a more straightforward and efficient implementation of array operations.

2. Consistency: By starting indexing from 0, the index directly corresponds to the offset from the beginning of the array. This consistency makes it easier to reason about array operations and avoids confusion.

3. Mathematical convenience: Starting indexing from 0 aligns with the mathematical concept of zero-based counting, which is widely used in computer science and mathematics.

4. Compatibility: Many programming languages, including C, C++, Java, and Python, follow the convention of starting array indexing from 0. This consistency across different languages allows for easier code portability and reduces confusion when working with arrays in different contexts.

Therefore, the correct answer to the given question is option A, which states that the index of the first element in an array is 0.