What type of lesion develops on the penis during primary syphilis?

The correct answer is a chancre, which is the primary lesion associated with primary syphilis. A chancre is a firm, round, and painless sore that appears at the site where the Treponema pallidum bacterium enters the body, typically on the genitalia, anus, or mouth. This sore is usually single but can be multiple, and it forms about three weeks after exposure to the infection.

In terms of the other options, while an ulcer may describe a broader category of lesions, it lacks the specific characteristics and the context of the primary lesion in syphilis. Similarly, while warts can appear on the penis due to other infections, such as human papillomavirus (HPV), they are not associated with primary syphilis. The term "lesion" is too generic to describe the specific type of lesion that is indicative of primary syphilis. Thus, the use of "chancre" is precisely accurate to denote the unique and defining feature of stage one of this sexually transmitted infection.