Covalent molecules have low melting and boiling point because:

Correct option is B

Correct option: (b), they have weak intermolecular forces

Covalent molecules generally have low melting and boiling points because:

• ​Weak Intermolecular Forces: In covalent compounds, the atoms are held together by strong covalent bonds within the molecules. However, the forces between individual molecules (called intermolecular forces) are relatively weak. These can include London dispersion forces, dipole-dipole interactions, or hydrogen bonds (if applicable). Since these forces are weaker than the covalent bonds, it takes less energy to separate the molecules, resulting in lower melting and boiling points.
  
• Molecular Structuresmall molecules: Most covalent compounds are composed of (such as water, oxygen, carbon dioxide). These smaller molecules have less mass and require less energy to move apart compared to large, complex molecules or ionic compounds.

In contrast, ionic compounds and giant covalent structures (like diamond or graphite) have much higher melting and boiling points because they involve stronger bonds or structures that require much more energy to break.