When the absolute value of the difference in electronegativity of two adjacent atom in a compound is 1, they are most likely bonded by .

polar covalent bond

Explanation:

Compounds are formed ether by donating, receiving or sharing of electrons making the bonds between them to  be ionic, or covalent .

Now  according to the rule  of type of bond assignment due to electronegativity difference of atoms

---When the  electronegativity difference is less than 0.5,  the bond formed is nonpolar covalent.

---When   the  electronegativity difference is between 0.5 and 1.6, the bond is considered polar covalent

---When  the  electronegativity difference greater than 2.0, then the bond is ionic.

Therefore, When the absolute value of the difference in electronegativity of two adjacent atom in a compound is 1, they are most likely bonded by  polar covalent bond eg HCl bond , CCL4 bond