represents a polar molecule.
Further Explanation:
The attraction between atoms, molecules or ions responsible for the formation of chemical compounds is a chemical bond. It is formed either due to electrostatic forces or by the sharing of electrons.
The bond that results from the complete transference of electrons between metals and non-metals is an ionic bond. Metals, being electropositive in nature, have the tendency to lose electrons whereas non-metals tend to gain electrons due to their high electronegativity. Cations are formed by metal atoms while non-metals form anions with the gain of electrons.
The bond formed by mutual sharing of electrons between bonded atoms is a covalent bond. Such bonds usually exist between two or more non-metals.
The polarity of a bond depends on the electronegativity difference between the bonded atoms. The more electronegative atom will attract electrons more towards itself, thereby developing negative charge on itself and leaving the less electronegative atom with positive charge. Symmetry also plays an important role to determine whether a molecule will be polar or not. If symmetry is present in a molecule it will be non-polar, even there is an electronegativity difference between the bonded atoms.
(1) 
This is formed by the combination of two Br atoms so there is no electronegativity difference between the bonded atoms and therefore this molecule is non-polar in nature (For structure, refer to the attached image).
(2) 
This molecule is composed of one carbon and two oxygen atoms but is symmetrical in nature so dipole moment gets cancelled out and therefore is non-polar in nature (For structure, refer to the attached image).
(3) 
This molecule consists of one carbon and four hydrogen atoms but is symmetrical in nature so dipole moment gets cancelled out and therefore is non-polar in nature (For structure, refer to the attached image).
(4)
This molecule is composed of one nitrogen and three hydrogen atoms. But N is more electronegative than H so this is polar molecular due to electronegativity difference. Also, this is unsymmetrical molecule (For structure, refer to the attached image) and therefore it is polar in nature.
Learn more:
Identification of ionic bonding: Which is a covalent compound?
Answer details:
Grade: High School
Subject: Chemistry
Chapter: Ionic and covalent compounds
Keywords: polar, non-polar, CH4, CO2, Br2, NH3, symmetrical, dipole moment, chemical bond, ionic bond, covalent bond.
