${\mathrm{AlF}}_3$ is more ionic than ${\mathrm{AlCl}}_3$. Explain?

Considering $\mathrm{x}$-axis as the internuclear axis, which out of the following will not form a sigma bond and why?

(a) $1\mathrm{s}$ and $1\mathrm{s}$ (b) $1\mathrm{s} \mathrm{and} 2{\mathrm{p}}_{\mathrm{x}}$ (c) $2{\mathrm{p}}_{\mathrm{y}} \mathrm{and} 2{\mathrm{p}}_{\mathrm{y}}$ (d) $1\mathrm{s} \mathrm{and} 2\mathrm{s}$.