Which statement best defines direct variation?

• A. A situation in which $y$ and $x$ are unrelated but accidentally show similar numerical trends over a small sample
• B. A quadratic relationship where $y$ depends on the square of $x$, and the constant $k$ changes with $x$
• C. A relationship where $y$ equals $k$ divided by $x$ for all nonzero $x$, meaning one variable increases while the other decreases, and they never change in the same direction under any circumstances
• D. A linear relationship of the form $y=kx$, where $k$ is a fixed constant, and $y$ and $x$ increase or decrease together

Answer: (D)