That response is incorrect - NONE of the answers were correct.

The correct cause of ECC is:

ECC is caused by a complicated combination of factors including the transmission of the infectious microbes (Mutans streptococci) and the age of the infant or child when they are exposed to the MS, diet and nutrition (including inappropriate feeding practices - particularly the use of baby bottles with a sugar-containing liquid), mucosal immunity, the relative susceptibility of the incisors and molars, and the early detection of these caries.