The correct option to fill in the blank is "for". In English, when expressing a duration of time, the preposition "for" is used. Here, the sentence describes the period she has been working, which is a span of five years. The use of "for" indicates that this activity took place over this entire time span.
Let's look at the options and their typical usage:
Thus, the sentence is correctly completed as: "She has been working here for five years."
The manager asked the team to complete the project _______ the end of the week.