White blood cells are colourless cells in the blood. They are part of the immune system and help fight infections.

Here is a simple explanation:
- White blood cells (WBCs) do not have a fixed color because they lack the red pigment found in red blood cells.
- They appear clear or colourless under a microscope.
- WBCs protect the body by attacking bacteria, viruses, and other harmful invaders.

Other cells:
- Red blood cells (RBCs) are red because they contain hemoglobin, which carries oxygen.
- Platelets help in blood clotting but are small and colorless or pale.
- Monocytes are a type of white blood cell, so they are also colourless.

So, the correct answer is White blood cells.

✅ Final Answer: White blood cells.