For this we question, you are looking for the best answer not the logical one. You have to go by process of elimination.
a) All species grow well at pH 7.
This is not true because the graph doesn't show all species.
b) This type of bacteria can grow well at a pH of 7.5. In our graph, you'd guess it would do well based on the high results of pH 7 and 8. But move to the next one to make sure there is not a better answer.
c) A pH of 2 is not listed on the chart do rule this option out.
d) Other types of bacteria are not shown on the graph, so you cannot make a blanket statement if the graph doesn't refer to other species.
The most correct answer is b.
