Which of these is a cocci occurring in single or pairs?

The correct option is Diplococci.

To understand why this is the correct answer, let's break down the classification of spherical bacteria (cocci) based on their cellular arrangement:
1. Cocci (singular: Coccus): These are spherical or oval-shaped bacteria. When these cells divide, they can remain attached to one another, forming characteristic arrangements depending on the plane of cellular division.
2. Diplococci: The prefix "diplo-" means double or pairs. Diplococci are spherical bacterial cells that divide in a single plane and remain attached to each other, characteristically occurring as pairs (or sometimes as single cells). Examples include Neisseria gonorrhoeae and Streptococcus pneumoniae.
3. Streptococci: The prefix "strepto-" means twisted chain. These cocci divide in a single plane but remain attached to form long chains of cells, rather than just pairs or single units.
4. Tetracocci (or Tetrads): The prefix "tetra-" means four. These cocci divide in two perpendicular planes, resulting in a group of four cells arranged in a square shape.

Therefore, cocci that occur primarily in single cells or pairs are classified as Diplococci.