In case of a linear isotropic dielectric, when placed in an external electric field, the induced dipole moment is

CONCEPT:

• A dielectric develops a net dipole moment in the presence of an external field E. The dipole moment per unit volume is called polarization and is denoted by P.

where χe is a constant. It is the characteristic of the dielectric and is known as the electric susceptibility of the dielectric medium.

Electric susceptibility

• Electric susceptibility can also be expressed in terms of dielectric constant K as

EXPLANATION:

• When a dielectric is placed in an external electric field, charges are induced, and a net dipole moment is formed.
• The dipole moment per unit volume is defined as polarisation.
• ​Linear isotropic dielectrics are defined as dielectrics for which
  • The direction of polarisation is the same as that of the external electric field. 
  • The strength of polarization is proportional to the external electric field. 
• Therefore option 1 is correct.