Fungus are more likely to grow at low pH and so they love acidic environment to grow and live. The organisms who love to grow in an acidic environment are called acidophiles.
The optimum pH at which fungi like to grow is between 5 to 6. Fungi growth condition is different from most of the microorganisms because micro-organisms tend to grow well at and above pH 7. So bacteria basically like alkaline environment to live.
When pH decreases from 7 to 5 the growth of fungus in increased by many folds which shows that fungus like to grow at low pH. Therefore the correct answer is b. 5