Which of the following orbital combinations form a sigma $(\sigma)$ bond? $1s-1s, 2s-2s, 1s-2s, 2p_x-2p_x, 2p_y-2p_y, 2p_z-2p_z$.

(A) $\sigma$-bond is formed by the head-on (axial) overlap of atomic orbitals.
$1$. $1s-1s$ overlap: Forms a $\sigma$-bond.
$2$. $2s-2s$ overlap: Forms a $\sigma$-bond.
$3$. $1s-2s$ overlap: Forms a $\sigma$-bond.
$4$. $2p_z-2p_z$ overlap (assuming $z$-axis as the internuclear axis): Forms a $\sigma$-bond.
$2p_x-2p_x$ and $2p_y-2p_y$ overlaps result in $\pi$-bonds because they overlap laterally (sideways).