Which of the following best defines a differential equation?

Solve the initial value problem: dy/dx = 3x^2, y(0) = 4.

Consider the velocity function $v\left(t\right)=3t-5$. If the initial position $s\left(0\right)=2$, determine the position function $s\left(t\right)$.

Solve the initial value problem: $\frac{d^{2}y}{d{x}^2}=2e^{2x}$, $y^{\prime }\left(0\right)=3$, $y\left(0\right)=1$.

Solve the initial value problem: $xdy-3\sqrt{y}dx=0$, $y\left(1\right)=4$.