Polar molecules are the molecules :

The correct option is: "Having a permanent electric dipole moment"

To understand why this is the correct answer, let's break down the nature of polar molecules step-by-step:

1. Definition of Polar Molecules:
A polar molecule is one in which the spatial distribution of positive and negative charges is not symmetric. This asymmetry typically arises from differences in electronegativity between the bonded atoms, causing a shift in electron density. As a result, one end of the molecule carries a partial positive charge, while the other end carries a partial negative charge.

2. Concept of Electric Dipole Moment:
An electric dipole consists of two equal and opposite charges (+q and -q) separated by a small distance (d). The electric dipole moment is a measure of the polarity of the system and is defined as:
$$p=q\times d$$
Because the centers of positive and negative charges in a polar molecule do not coincide even in the absence of an external electric field, these molecules inherently possess a non-zero, intrinsic, or permanent electric dipole moment. Examples include water (H₂O) and hydrochloric acid (HCl).

3. Evaluation of Other Options:
• Having zero dipole moment: This describes non-polar molecules (like O₂ or CO₂) where the centers of positive and negative charges coincide, canceling out any dipole moment.
• Acquire a dipole moment only in the presence of electric field due to displacement of charges: This describes non-polar molecules that undergo polarization under an external electric field (induced dipole moment), rather than polar molecules which already have a permanent one.
• Acquire a dipole moment only when magnetic field is absent: Magnetic fields do not determine the fundamental electrostatic definition or existence of a permanent electric dipole moment in polar molecules.