What do you think the old stuff and the new stuff are in the chemical reaction that makes a sparkler light up? IQWST

What types of atoms typically form covalent bonds?

The correct

d) Nonmetals with nonmetals, because their difference in electronegativity is below 1.7.

En example to explain:
An example of a covalent bonding: HCl -> 2 nonmentals
-> a difference in electronegativity less than 1.7:

EN(Cl) = 3.0 and EN (H) = 2.1 (you can search these values in a periodic table)
/\EN = 3.0 - 2.1 = 0.9

0.9 < 1.7

I hope this helped you out!