These are colourless, disc-shaped blood cells without nuclei.

The correct option is C

blood platelets

The correct option is (C).

Explanation of the correct option:

Platelets:

1. Platelets are disc-shaped, colourless cells.
2. They are devoid of nucleus.
3. They help in the clotting of blood preventing excessive loss of blood.
4. These are the smallest of all cells present in the blood.

Explanation of the incorrect options:

Option A:

RBCs are bi-concave, red-coloured cells, that are devoid of nucleus.

Option B:

WBCs are spherical, colourless cells. They are nucleated.

Final answer: Blood platelets are colourless, disc-shaped blood cells without nuclei.