Which of the following is NOT one of the four relational operators?

The option "Increment by" is not considered one of the four relational operators. Relational operators are used to compare two values and produce a boolean outcome, typically true or false. The common relational operators include "less than," "equal to," and "not equal to." These operators help in making decisions in programming, algebra, and other areas where comparison of values is necessary.

On the other hand, "increment by" is an action often used in programming to increase the value of a variable by a certain amount, but it does not perform a comparison between two values. This places it outside the definition of relational operators, which are specifically concerned with the relationship between values rather than manipulating them. Thus, "Increment by" does not fit the category of relational operators.