In a simple harmonic motion, which of the following quantities are...

In simple harmonic motion, the restoring force $\vec{F}$ is proportional to the displacement $\vec{r}$ from the mean position and is directed opposite to it.

$\vec{F} = -k\vec{r}$

By Newton's second law, the acceleration $\vec{a}$ is given by:

$\vec{a} = \dfrac{\vec{F}}{m} = -\omega^2\vec{r}$

Since $\vec{F}$, $\vec{a}$, and $\vec{r}$ are always collinear (either parallel or anti-parallel to each other), the cross product of any two of these vectors is always zero.

$\vec{F} \times \vec{a} = \vec{0}$

$\vec{F} \times \vec{r} = \vec{0}$

$\vec{a} \times \vec{r} = \vec{0}$

The velocity $\vec{v}$ and displacement $\vec{r}$ are collinear in linear SHM, so their dot product $\vec{v} \cdot \vec{r}$ is not always zero (it is zero only at the mean and extreme positions).