Which statement best defines direct variation?

Which of these accurately describes inverse variation?

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 $p=25\text{ kPa}$.

Determine whether the pairs $(2,12)$, $(3,8)$, $(4,6)$ are examples of direct variation, inverse variation, or neither.

A gardener finds that the required fertilizer mass $y$, in units of $\text{kg}$, will vary directly with the garden area $A$, in units of $\text{m}{}^2$, and will vary inversely with the fertilizer concentration $c$, in units of $\frac{\text{kg}}{{\text{m}}^2}$ (for the recommended strength), such that $y=\frac{kA}{c}$. If the gardener uses 50 $\text{kg}$ of fertilizer for a 200 ${\text{m}}^2$ plot with a concentration of $c=2$, determine how many kilograms will be needed for a 300 ${\text{m}}^2$ plot at concentration of $c=3$.