Which type of bacteria form chains and may cause illnesses such as strep throat?

The type of bacteria that form chains and can cause illnesses like strep throat is Streptococci. These bacteria are typically arranged in a chain-like formation, which distinguishes them from other types of cocci. Streptococci are known to be responsible for various infections, including strep throat, which is an infection of the throat and tonsils caused by the group A Streptococcus bacteria.

Staphylococci, on the other hand, form clusters or bunches similar to grapes and are associated with different types of infections, such as skin infections. Diplococci consist of paired cocci, usually seen in linked pairs, and are notably involved in infections such as pneumonia. The term 'cocci' is a broad classification for spherical-shaped bacteria and doesn't specify the arrangement or types, making it less relevant when identifying the specific type of bacteria associated with strep throat. Therefore, the arrangement and characteristic of the bacteria guide us to choose Streptococci as the correct answer.