Which of the following terms is in its singular form?

The term "bacillus" is indeed in its singular form. A bacillus refers to a specific type of rod-shaped bacterium, and in microbiology, this term is used to denote a single organism of that genus.

In contrast, "bacteria" is the plural form of bacterium, indicating multiple organisms. "Scapulae" is also a plural form, referring to the shoulder blades; the singular form is "scapula." Similarly, "veins" is the plural form of "vein," which refers to the blood vessels that carry blood towards the heart. Thus, bacillus stands out as the only term in the options given that represents a single entity.