U see according to fazan rule smaller cation forms a bond which more covalent in nature due to high latice enthalpy. and as we move along the period metallic character decreases due to increase in valence electron and as metallic character decreases, the bond formed by this becomes more covalent.
So, BeH2 is more covalent than the others given in the option.
i hope u get it.

Significant covalent character in hydrides is determined by the electronegativity difference between the elements involved. The higher the electronegativity difference, the more ionic the bond and the lower the covalent character. On the other hand, a smaller electronegativity difference indicates a more covalent bond.

(A) LiH:
- Lithium (Li) has an electronegativity of approximately 1.0, while hydrogen (H) has an electronegativity of approximately 2.2.
- The electronegativity difference is 1.2, indicating an ionic character.
- Therefore, LiH has less covalent character.

(C) MgH2:
- Magnesium (Mg) has an electronegativity of approximately 1.2, and hydrogen (H) has an electronegativity of approximately 2.2.
- The electronegativity difference is 1.0, indicating an ionic character.
- Therefore, MgH2 also has less covalent character.

(D) All:
- If we consider all the options, we can see that LiH and MgH2 both have ionic character.
- Therefore, option (D) cannot be the correct answer.

(B) BeH2:
- Beryllium (Be) has an electronegativity of approximately 1.5, and hydrogen (H) has an electronegativity of approximately 2.2.
- The electronegativity difference is 0.7, indicating a more covalent character.
- The smaller electronegativity difference suggests that BeH2 has significant covalent character.

Therefore, the correct answer is option (B) BeH2, as it has significant covalent character due to a smaller electronegativity difference between beryllium and hydrogen.