The correct way to declare a string variable in C is option 'A': string s;

Explanation:

In C, a string is typically represented as an array of characters. There are several ways to declare a string variable, but the most common and recommended way is to use the string keyword.

1. char s[];

This declaration creates a character array named 's'. However, it does not specify the size of the array. To use this array as a string, you would need to initialize it with a null-terminated string.

2. char* s;

This declaration creates a character pointer named 's'. While it can be used to point to a string, it does not allocate memory for the string itself. Therefore, you would need to allocate memory dynamically or assign it to a valid memory address before using it as a string.

3. string* s;

This declaration creates a string pointer named 's'. However, the string keyword is not a standard data type in C. It may be used in some libraries or frameworks, but it is not a part of the C language itself.

4. string s;

This declaration creates a string variable named 's'. The 'string' keyword is not a built-in data type in C, but it can be defined using typedef. By convention, the 'string' type is used to represent a null-terminated character array, which is commonly used to store and manipulate strings in C.

Therefore, option 'A': string s; is the correct way to declare a string variable in C. It declares a string variable named 's' that can be used to store and manipulate strings without requiring additional memory allocation or initialization.