The only bonds in a formula unit of caf2 are nonpolar covalent polar covalent ionic metallic

ionic

Explanation:

Ionic bonds exist between Ca and F ions in CaF₂. Ionic bonds are interatomic bonds formed by the transfer of electrons from one atom to the other.

The donor atom here is Ca and it has two valence electrons. Fluorine is the receiving atom with 7 electrons in its outermost shell.

Ca would give one each of its two outermost electrons to the fluorine atoms to complete their octet. Ca ion would now resemble Argon and the flourine atoms would look more like Neon atoms.

This is an ionic bond

the best answer is d because the strong cohesion is caused by the ability of water molecules to undergo hydrogen bonding.

a is incorrect because when water solidifies, it becomes   ice, which expands and occupies a bigger volume for the same mass of water in its liquid state.

b is incorrect because water has high surface tension. besides bonding to itself it hydrogen bonds to other molecules at the top decreasing possibility for breaking of attractive forces.

c is incorrect because the water molecule has oxygen which is highly electronegative and so pulls the electron pair towards itself creating a dipole moment, making the molecule polar.

e is incorrect because water has strong adhesion, ability to stick to other molecules such as glass because of this molecules having stronger hydrogen bonding than water.