A. You cannot copy slides between presentations
B. Use File → Import Slides only
C. Copy slide from source → Paste into destination → Slide is added
D. Save the slide as a template first

Solution

C. Copy slide from source → Paste into destination → Slide is added

Explanation

Slides can be copied from one presentation and pasted into another. एक presentation की slide को copy करके दूसरी presentation में paste किया जा सकता है।