Answer: AD < BD < AC< AB
Explanation:-
Electronegativity is defined as the property of an element to attract a shared pair of electron towards itself.
Lower is the electronegativity diffrence, more will be the covalent character. Higher is the electronegativity diffrence, more will be the ionic character.
1. AB: Electronegativity difference = electronegativity of A - electronegativity of B = 3.8-3.3= 0.5
2. AD: Electronegativity difference = electronegativity of A - electronegativity of D= 3.8-1.3= 2.5
3. BD: Electronegativity difference = electronegativity of B - electronegativity of D= 3.3-1.3= 2.0
4. AC: Electronegativity difference = electronegativity of A - electronegativity of C= 3.8-2.8= 1.0
Thus the order of increasing covalent character is AD < BD < AC< AB.