Whereby means:

• A. By which
• B. Through which
• C. Because of which
• D. With which

Answer: (A)

Whereby signals the method or mechanism by which something happens. It is essentially a formal way of saying “by which” (or sometimes “through which”) that explains the means used to achieve something. In this sense, the phrase focuses on the way something is accomplished. For example, “They devised a system whereby users can log in” means “a system by which users can log in.” That makes the option meaning “by which” the best fit. The other phrases would shift the sense: “because of which” points to a cause, and “with which” suggests an instrument; “through which” is sometimes close but isn’t the precise gloss of whereby in this usage.