Which organism typically forms clusters and is classified as gram-positive?

Staphylococcus is the correct answer because it is characterized by its unique clustering arrangement of cocci (spherical bacteria) that resemble grape-like clusters. This characteristic is a result of the way the bacteria divide in multiple planes. Additionally, Staphylococcus is classified as gram-positive, as it retains the crystal violet stain used in the Gram staining procedure, appearing purple under a microscope.

In contrast, Streptococcus typically forms chains or pairs of cocci, not clusters, and is also gram-positive. Bacillus and Clostridium are both rod-shaped (bacilli) organisms; while Bacillus can be gram-positive, Clostridium is also gram-positive but does not form clusters and is predominantly known for being anaerobic. Therefore, the defining feature of clustering in Staphylococcus makes it the organism typically associated with such a description.