Normal flora can become pathogenic primarily when B. displacement occurs to sterile body sites.
When normal flora, which normally reside in certain areas of the body (like the gut or skin), are introduced to normally sterile areas (such as the bloodstream or internal organs), they can potentially cause infections. Other factors like a compromised immune system or injury can also influence pathogenicity, but the correct answer relevant to your options regarding normal flora becoming pathogenic is option B.
Options A and C generally relate to decreased risk of infection by normal flora, and option D (intact skin) also suggests that the body's defenses are working to keep pathogens, including opportunistic normal flora, from causing harm.