The correct answer is option 4: Compact fluorescent lamp (CFL). CFLs are energy-saving lighting options that contain a gas-filled tube and a small amount of mercury vapor. When electricity is supplied, the mercury vapor inside the CFL emits ultraviolet (UV) light. This UV light interacts with the phosphor coating on the inside of the tube, causing it to emit visible light.

Option 1, "Condensed filament light," is not the correct full form of CFL. It is important to note that CFLs do not use filament technology like traditional incandescent bulbs.

Option 2, "Compact filament lamp," is also not the correct full form of CFL. Again, CFLs do not use filaments.

Option 3, "Condensed fluorescent lamp," is not the correct full form either. Although it incorporates the term "fluorescent lamp," it is not the commonly used acronym for CFL.

It is crucial to choose energy-efficient lighting options like CFLs to save electrical energy and provide sufficient light. CFLs use much less energy compared to traditional incandescent bulbs, making them a more environmentally friendly choice.