Bacterial morphology: the cocci that most commonly occur as single cells or as paired cells are called:

Introduction / Context:
Cocci are spherical bacteria that display characteristic arrangements based on the planes of division and whether daughter cells remain attached. Recognizing typical arrangements helps with rapid microscopy-based identification.

Given Data / Assumptions:

• Single cocci and pairs (two attached cocci) are commonly observed.
• Chains indicate repeated division in one plane with cells remaining attached.
• Packets (tetrads, sarcinae) result from division in perpendicular planes.

Concept / Approach:
The term diplococci refers to pairs of cocci. Examples include Neisseria species (kidney bean-shaped diplococci). Single cocci may be seen as well, but the named arrangement referring to pairs is diplococci. Chains of cocci are termed streptococci; groups/grapelike clusters are staphylococci.

Step-by-Step Solution:

Verification / Alternative check:
Gram-stained smears: pneumococci (Streptococcus pneumoniae) often appear as lancet-shaped diplococci after division in a single plane with pairing.

Why Other Options Are Wrong:
Streptococci: Chains, not pairs. Tetracocci: Packets of four from division in two planes. None of these: Incorrect because diplococci is the accepted term.

Common Pitfalls:
Confusing diplococci with streptococci; remember that strepto- implies chains.

Final Answer:
Diplococci