The figure above shows the historic composition, as percentages, of

Official Explanation

For the first question, you first need to understand what you're being told to look for. Because you're looking for the first point at which the bacteria couldn't survive, you're looking for the point at which oxygen makes up more than 10% of the atmosphere. Notice that while oxygen first starts to be a noticeable part of the atmosphere around 2.4 billion years ago, that it's between 2.4 and 2 billion years ago that it is a big enough part of the atmosphere to cause major issues for the bacteria. This time scale is solidly within the Proterozoic, which is the correct answer.

For the second question, it is important to look at what you are told about the information given in the graph. The graph gives you information about the percent of each gas in the atmosphere - nothing about the sheer amount of gases. So while the percent of the atmosphere that nitrogen accounts for decreases, you have no indication of whether that is because other gases are added or because the amount of nitrogen decreases - the answer to this question is therefore "uncertain".

For the third question, you are looking for a trend within the atmosphere that corresponds to the use of atmospheric carbon dioxide to produce oxygen. There are two possible places for this: where carbon dioxide starts to decrease and where oxygen starts to increase. While it is true that carbon dioxide decreases around 4.3 billion years ago, no oxygen is added to the atmosphere. When oxygen does begin to be produced, there is carbon dioxide present in the atmosphere (and the percent of carbon dioxide in the atmosphere does decrease slightly). This event occurs around 2.4 billion years ago.