Which characteristic defines a DSLR camera?

A DSLR (Digital Single-Lens Reflex) camera is defined primarily by its unique optical design, which includes a movable mirror mechanism. This mirror reflects light from the camera's lens upwards into an optical viewfinder, allowing photographers to see precisely what the lens is capturing in real-time. This direct viewing through the lens is a key advantage because it enables accurate framing and the visualization of depth of field before the shutter is released.

The other options do not accurately represent the characteristics of a DSLR camera. While some cameras may have fixed lenses or be lighter compared to DSLRs, these features are not inherent to DSLRs themselves. Additionally, DSLRs do use electronic sensors to capture images, distinguishing them further from older film-based cameras. Thus, the presence of a movable mirror, enabling direct viewing, is what fundamentally defines a DSLR camera.