Bacteria in general grow optimally when the pH is around 7 (5-9 let's say). That is not to say that there are not bacteria that grow in extremely acidic conditions, like in our stomach, or others in extremely basic conditions, like the conditions near volcanoes. Nonetheless, bacteria that grow in food prefer acidic environments over basic ones and usually cannot tolerate the acidity of fruits which is around 4 pH for the most acid ones. For example, stuff like milk and bread that have a pH from 5-6 are prime examples of food where bacteria would thrive. Thus, the correct answer would be b.