Syphilis is caused by
(a) Treponema pallidum
(b) Leptospira
(c) Pasteurella
(d) Vibrio

Hint: Syphilis is a contagious disease which mostly spread through sexual contact. If you had unprotected sex, oral sex and anal sex, you have higher chances of getting affected by syphilis. It can pass through the infected person by any means.

Complete step by step answer:
Syphilis is caused by Treponema pallidum. Syphilis can be caused through the sore on the infected person’s body when it comes to direct contact. In the initial stages of Syphilis, it causes a sore on the rectum, anus and around the mouth. In the second stage, it is characterized by a rash and the final stage does not show any symptoms and can lead to brain damage and damage to the nerves, eyes and heart.
So, the correct answer 'Treponema pallidum'.

Additional information:
Leptospira: It is a flexible gram-negative spirochete which is spiral-shaped and causes leptospirosis which may result in kidney or liver failure. It can be spread by contact with the urine of an infected animal or in contact with soil or food contaminated with the urine of an infected animal.
Pasteurella: It is an anaerobic gram-negative bacteria. Pasteurella is pathogenic and can cause infection by domestic animal’s bite like cats and dogs. Pasteurella causes Pasteurellosis in humans which is a local wound infection caused by the bites of domestic animals.
Vibrio: Vibrio is an anaerobic gram-negative bacteria. It can cause foodborne infection which is associated with undercooked seafood. It is a comma-shaped bacteria which causes cholera. It is caused by the bacteria which releases toxin and spreads through the faeces of an infected person. This occurs in underdeveloped countries and places where there is poor sewage disposal.

Note: Syphilis is a sexually transmitted disease (STD) which is caused by bacteria and can cause severe health issues if not treated well. It is often known as the 'great imitator' as it shows several signs and symptoms of the disease. Syphilis is considered as a foreign disease as it was first spread by the soldiers who prostituted in other countries.