C Certified Entry-Level (CLE) Programmer Certification Practice Test

The function used to calculate the length of a string in C is `strlen()`. This function is part of the standard C library, specifically included in the `<string.h>` header file. When called, `strlen()` takes a string (character array) as an argument and returns the number of characters in that string, excluding the null terminator (`\0`) that marks the end of the string.

For example, if you have a string defined as `char str[] = "Hello";`, calling `strlen(str)` would return 5, since there are five characters in "Hello". It is important to understand that `strlen()` counts only the characters and not the null character, which is crucial for proper string manipulation in C.

The other options do not correspond to any standard C library functions for calculating string lengths. This makes `strlen()` the only correct function in the context of obtaining the length of a string in C programming.