The correct answer is choice (3). Having an octet configuration is an extremely stable condition. During the formation of a chemical bond, the elements tend to acquire the stable octet configuration of the nearest noble gas.  

The correct answer is choice (3).

A molecule of water has two lone pairs of electrons.

The correct answer is choice (3). In a molecule of carbon monoxide (CO), there is a triple bond between carbon and oxygen atoms which comprises of two covalent bonds and one coordinate bond. The octets of carbon and oxygen atoms are not complete.

The correct answer is choice (1). Carbon is tetravalent and its compounds are essentially covalent.

Covalent compounds are formed between non-metals which do not have free electrons. So, these are poor conductors of electricity. They form covalent bonds which are weaker than ionic bonds. They are insoluble in water and dissolve in non-polar solvents.

The element X has 2 electrons in its valence shell. So, X is a metal and it will prefer to lose 2 electrons. The element Y has 6 electrons in its valence shell. So, Y is a non-metal and it will prefer to gain 2 electrons. Therefore, the compound between X and Y is ionic and will have the formula XY.                                                                                                            

Except CH₄, all others are ionic compounds. CH₄ is a covalent compound. 

Aluminium and fluorine will combine to form an ionic bond because it is a bond between a metal and a non-metal.

MgCl₂ is an ionic compound and the ions are held together by strong electrostatic forces of attraction. Therefore, MgCl₂ has the highest melting and boiling points.