Point defect in crystal lattice : Self interstitials
Linear defect in crystal lattice : Grain boundaries
Planar defect in crystal lattice : External surfaces
Volume defect in crystal lattice : Other phases

The correct answer is option B: Linear defect in crystal lattice - Grain boundaries.

Explanation:

Point Defects:
- Point defects refer to the defects that occur at a single or few lattice sites in a crystal lattice.
- These defects can be classified into two types: vacancy defects and interstitial defects.
- Vacancy defects occur when an atom is missing from its lattice site, leaving a vacant site.
- Interstitial defects occur when extra atoms occupy the interstitial sites between the regular lattice positions.

Linear Defects:
- Linear defects, also known as dislocations, occur when there is a deviation from the regular arrangement of atoms along a line or plane in the crystal lattice.
- They can be classified into two types: edge dislocations and screw dislocations.
- Edge dislocations occur when an extra half-plane of atoms is inserted into the crystal lattice, causing a misalignment of the atoms on either side of the dislocation line.
- Screw dislocations occur when the atoms are arranged in a helical pattern around the dislocation line.

Planar Defects:
- Planar defects refer to the defects that occur on a plane or surface within the crystal lattice.
- These defects can be classified into two types: grain boundaries and external surfaces.
- Grain boundaries occur when there is a discontinuity in the crystal lattice due to the presence of different crystal orientations or grains.
- External surfaces refer to the surfaces of the crystal lattice that are exposed to the external environment.

Volume Defects:
- Volume defects, also known as bulk defects, occur within the bulk of the crystal lattice.
- These defects can include impurities, second phases, or voids within the crystal structure.
- Impurities are foreign atoms or ions that are incorporated into the crystal lattice.
- Second phases refer to the presence of a different crystal phase within the same material.
- Voids are empty spaces or vacancies within the crystal lattice.

In the given options, the pair "Linear defect in crystal lattice: Grain boundaries" is not correctly matched because grain boundaries are planar defects, not linear defects. The correct pair for linear defects would be "Linear defect in crystal lattice: Dislocations" which include edge and screw dislocations.