Grain boundaries are the examples of  A. Vacancies B. Screw dislocations C. Planar defects D. None of these

Grain boundaries are examples of Planar defects. So, the answer is C. Planar defects.

Here's a step-by-step explanation:

1. In materials science, defects are irregularities that occur in the ideal ordered arrangement of atoms in a crystalline material. These defects can be grouped into three categories: point defects, line defects, and planar defects.

2. Vacancies (option A) and screw dislocations (option B) are examples of point and line defects, respectively. They are not the correct answer because they do not describe grain boundaries.

3. Grain boundaries (the subject of the question) occur where the crystallographic direction of the lattice changes. This is a two-dimensional defect, and hence, it falls under the category of planar defects.

4. Therefore, the correct answer is C. Planar defects.

5. Option D is incorrect because grain boundaries are a type of defect in the crystal structure, so they are not "none of these".