Check the answer that best describes the relationship between f(x) and x. (for example if f(x) is θ(x)θ(x) check that as your answer and not o(x)o(x) or ω(x)ω(x) even though these are true also in this case.)

Which course is this for? I feel like I may need a little more information, but the simplest explanation I can give given the question is:

f(x) is the value of a function f evaluated at the value x. The formal definition of a function is a particular set of rules that associates a particular value with another. For an analogy: you can think of f as a machine that takes in an input, transforms it according to a certain set of rules, and produces an output. In this analogy, x would be the input and f(x) would be the output.

A. is the answer in this question