This is a competency based question used to determine your level of expertise in statistical charting. Since traffic growth is charting two axes, 'traffic', an integer, and 'time,' a continuum, the best option here is a line graph. It would also be correct to opt to use a bar graph, with the bars representing fixed quantities of time (like Day or Month). Most of all, when you answer, avoid saying 'a pie chart' or 'I don't know any types of charts.' If this question does not apply to your experience as a Marketing Assistant, focus your answer more on your desire to learn.