Step 1: Sort defects by dimension.
Zero dimensional defects are points, like vacancies. One dimensional defects are lines, like dislocations. Two dimensional defects are planes, like stacking faults and grain boundaries.
Step 2: Test the dislocation statements.
Both screw and edge dislocations are line defects, since the lattice distortion runs along a line in the crystal. So calling a screw dislocation a planar defect, as option A does, is wrong. Option B calls edge dislocation a line defect, which matches, so it is correct.
Step 3: Test the stacking fault statement.
A stacking fault breaks the regular layer sequence over a sheet of atoms, so it belongs to the planar defect group, not the line defect group. Option C is wrong.
Step 4: Test the vacancy statement.
A cation vacancy is one missing ion at one lattice site, which fits the point defect definition exactly. Option D is correct.
Step 5: Conclude.
Only B and D describe the defects correctly.
\[ \boxed{\text{(B) and (D) are correct}} \]