$v_2$ and $v_3$
Work Step by Step
If a vector $v$ can be obtained by multiplying a vector $w$ by a constant, then $v$ and $w$ are parallel. If the dot product of two vectors is $0$ then they are orthogonal. $(-3i-6j)(-8i+4j)=(-3)(-8)+(-6)(4)=24-24=0$, hence $v_2$ and $v_3$ are orthogonal.