[hwstuff]

This was a problem on my homework that I got wrong. The other choices were bonds between atoms with very large electronegativity differences, so they were wrong. All answer choices had just "-" between the chemical symbols, so I don't know whether it means just a single bond or not.

[Fudjsk]

The larger the electronegative difference, the stronger the bond is. The smaller atom is more electronegative. If the atoms are the same then you compare bond type. From there you can compare which molecule s more electronegative than the other.

[Vidya]

This was a problem on my homework that I got wrong. The other choices were bonds between atoms with very large electronegativity differences, so they were wrong. All answer choices had just "-" between the chemical symbols, so I don't know whether it means just a single bond or not.

Bond lengths are compared on the basis of size of the orbitals overlapping in bond formation.
Like in C-C  .it is 2s and 2p orbitals participating in chemical bond formation after hybridization.
In Cl-Cl it is 3s and 3p orbitals and 3s ,3p orbitals are bigger in size than 2s and 2p so bond length in Cl-Cl is more than C-C.

[Babcock_Hall]

I looked up the bond dissociation energy of Cl-Cl, and I found 242 kJ per mole.  The bond dissociation energies of carbon-carbon single bonds show some variation in the table that I consulted but for sp³-hybridized carbon atoms, most fell between 360 and 380 kJ per mole.  Therefore, I think that the premise of your question is mistaken.

[Enthalpy]

The small bond energy between halogen atoms is what lets them bind with other elements.

H-H is as strong as H-Cl, but Cl-Cl is weak, so Cl₂ reacts with H₂.

H-H as strong as H-Cl tells that the electronegativity difference isn't important.

Some small tables there:
http://www.chem.ucalgary.ca/courses/350/Carey5th/useful/bonde.html
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Chemical_Bonding/Fundamentals_of_Chemical_Bonding/Bond_Energies