To understand what the code strcat(an_array, "This") does, we need to know that strcat() is a function in C language that concatenates (joins) two strings.

The function takes two arguments: the first argument is the destination array (where the concatenated string will be stored), and the second argument is the source string (which will be concatenated to the destination array).

In this case, the function call strcat(an_array, "This") concatenates the string "This" to the end of the string already stored in an_array. Therefore, the correct option is:

The Answer is: B. Adds "This" to the end of an_array