Which STD is responsible for the appearance of a chancre, a small, painless, reddish-brown sore on the mouth or on the sex organs?

The correct option is D

Syphilis

Syphilis is a bacterial disease caused by Treponema pallidum. It is transmitted by direct contact with an open sore or from infected mother to child.

Primary or early stages of syphilis manifests as red, painless ulcerations (or chancre) on genitals at the site of bacterial entry. These ulcerations become hard and deeper after which they resolve. After this, patients often notice enlargement of the lymph nodes in the groin region. In the secondary stage there are skin rashes all over the body, including palms, soles and around the mouth.