To answer this question, the user needs to know what string functions are and be familiar with some commonly used string functions.

String functions are functions that operate on a string and return a modified string or information about the string. Some examples of string functions include substring, upper/lower case, length, concat, and trim.

Now, let's go through each option and determine which one is not a string function:

A. concat: This is a string function that concatenates two or more strings together.

B. initcap: This is a string function that capitalizes the first letter of each word in a string.

C. round: This is not a string function. It is a mathematical function that rounds a number to a specified number of decimal places.

D. length: This is a string function that returns the length of a string.

Therefore, the answer is:

The Answer is: C