The answer is pneumonia. Milady defines diplococci precisely: they "are spherical bacteria that grow in pairs and cause diseases such as pneumonia." The "diplo-" prefix refers to the paired arrangement, distinguishing this bacterial shape/grouping from staphylococci (clusters) and streptococci (chains).
Strep throat, syphilis, and typhoid fever — the other choices on this question — are each caused by different bacteria entirely (streptococci, a spirochete, and Salmonella typhi, respectively), not diplococci. Barbers aren't expected to diagnose disease, but the exam does test recognizing which bacterial shape is associated with which named illness.
Milady Standard Barbering, 6th ed., Ch. 4 (Infection Control), p. 65: "Diplococci... are spherical bacteria that grow in pairs and cause diseases such as pneumonia."
This is 1 of the real Sanitation questions in FadeJunkie's Milady-sourced 300-question bank. See the Sanitation practice questions page for a fuller sample, or try the Sanitation practice quiz and flashcards for free. A one-time $15 lifetime pass unlocks the complete question bank across all 13 tested topics.
More real Sanitation questions with sourced answers: What Category Do Disinfectants Fall Under, What Organ Does Hepatitis Damage, What Should You Never Mix Bleach With, What Can't Pathogens Enter the Body Through, and 15 more on the Sanitation practice questions page.
Browse every topic on the Practice Question Hub, or see the Texas Barber State Board Exam Guide for the full exam process — required hours, PSI scheduling, and fees.