Which statement best defines direct variation?

Given that $y$ varies directly as $x$ and $y=12$ when $x=4$, find the equation relating $x$ and $y$.

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 $p=25\text{ kPa}$.

The time $T$ required to complete a fixed project varies inversely with the number of workers $n$. If $8$ workers complete the project in $15$ hours, how many workers are needed to finish the project in $10$ hours?

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?