Polar molecules are the molecules :

The correct option is: Having a permanent electric dipole moment

Let's break down the definition and characteristics of polar molecules:

1. What is a Polar Molecule?
A polar molecule is a molecule in which one end is slightly positive, while the other end is slightly negative. This uneven distribution of electron density occurs because of a significant difference in the electronegativity of the atoms involved in the covalent bonds, causing the more electronegative atom to pull the shared electrons closer to itself.

2. Permanent Electric Dipole Moment:
Because of this inherent and constant separation of positive and negative charge centers, polar molecules naturally possess a permanent electric dipole moment. A dipole moment is a vector quantity that measures the polarity of a system. In a polar molecule, this dipole moment exists intrinsically, even in the complete absence of any external electric field.

3. Analyzing the other options:
- Having zero dipole moment: This describes a non-polar molecule, where the spatial center of positive charge exactly coincides with the center of negative charge, canceling out any overall polarity.
- Acquire a dipole moment only in the presence of an electric field: This describes an induced dipole moment. When a non-polar molecule is placed in an external electric field, its electron cloud gets temporarily distorted, creating a temporary dipole, which disappears when the field is removed.
- Magnetic field: The formation of a permanent electric dipole moment is fundamentally related to the spatial distribution of electric charges (electrons and protons) within the molecule, and it does not depend on the absence or presence of a magnetic field.

Therefore, the defining characteristic of a polar molecule among the given choices is that it inherently possesses a permanent electric dipole moment.