Which statement best defines direct variation?

Given the direct variation $y=\frac{3}{4}x$, solve for $x$ when $y=12$.

If $y$ varies inversely as $x$ and $y=6$ when $x=2$, find the inverse variation equation and then compute $y$ when $x=3$.

If $y=kx$ and someone changes the unit of $x$ such that: $x^{\prime }=2x$, how must $k$ change such that $y$ will represent the same physical quantity in the new units of measurement?

According to Boyle's law, gas pressure $p$ varies inversely with volume $V$. If $p=150\text{ kPa}$ when $V=3\text{ L}$, determine the volume when $p=25\text{ kPa}$.