Explanation:

The correct answer is option B, which states that using the `concat()` function is not a valid method to concatenate two strings in C.

Concatenating strings in C:
In C, there are multiple ways to concatenate two strings. Concatenation refers to combining two strings into a single string. Here are some commonly used methods:

1. Using the `+` operator:
The `+` operator is used for arithmetic operations in C, but it can also be used to concatenate two strings. However, it does not modify the original strings and creates a new string with the combined result.

2. Using the `strcat()` function:
The `strcat()` function is a standard library function in C, defined in the `string.h` header. It is specifically designed to concatenate two strings. It modifies the original string by appending the second string to the end of the first string.

3. Using the `strncat()` function:
Similar to `strcat()`, the `strncat()` function is used to concatenate two strings. However, it allows specifying the maximum number of characters to be appended from the second string.

4. Using the `strcpy()` and `strcat()` functions:
The `strcpy()` function is used to copy one string into another, and the `strcat()` function is used to concatenate two strings. By combining these functions, we can achieve string concatenation.

Invalid method: concat()
The `concat()` function is not a standard library function in C. It does not exist in the C standard library, so it cannot be used to concatenate strings. It might be a custom function defined by a programmer, but it is not a part of the standard C language.

In summary:
To concatenate two strings in C, the valid methods are using the `+` operator, `strcat()`, `strncat()`, or a combination of `strcpy()` and `strcat()`. The `concat()` function is not a valid method in C for string concatenation.