What type of organism is characterized by being gram-positive and arranged in chains?

The organism characterized by being gram-positive and arranged in chains is Streptococcus. This genus consists of spherical bacteria (cocci) that typically form chains or pairs when they divide. Their arrangement is a key microscopic feature that helps differentiate them from other gram-positive bacteria.

Streptococcus is notable for its various species that can lead to different infections, including streptococcal throat infections, pneumonia, and skin infections. Their gram-positive nature indicates that they have a thick peptidoglycan layer in their cell walls, which retains the crystal violet stain used in the Gram staining process, resulting in a blue or purple appearance under the microscope.

In contrast, the other options represent different types of bacteria: Staphylococcus, while also gram-positive, generally forms clusters rather than chains; Bacillus is primarily rod-shaped and can form chains, but they are not typically described as chain-forming in the same way as Streptococcus; and Corynebacterium tends to appear as 'Chinese letters' or palisades under the microscope rather than in chains. Therefore, the distinct arrangement of Streptococcus in chains is the defining characteristic that makes it the correct answer.