Step 1: Understanding the Question:
A molecule is polar if it has a non-zero net dipole moment. This happens when the center of positive charge does not coincide with the center of negative charge due to asymmetry. Step 2: Detailed Explanation:
Looking at the provided diagrams:
(a) Represents a bent molecule (like $H_2O$). The dipole moments of the two bonds do not cancel each other, resulting in a net dipole moment. Hence, it is polar.
(b) Represents a homonuclear diatomic molecule (like $N_2$ or $O_2$). It is perfectly symmetric; the centers of charge coincide, so it is non-polar.
(c) Represents a linear symmetric molecule (like $CO_2$). The individual bond dipoles are equal and opposite, cancelling out to zero. It is non-polar.
(d) Represents a homonuclear diatomic molecule (like $H_2$). Similar to (b), it is non-polar. Step 3: Final Answer:
The molecule labeled (a) is the polar molecule. According to the options provided, this corresponds to choice (B).
