Could there be a problem with the interpretation here? The question asks for what's not valid. The third answer - Given that an email contains a big number, it is more likely to be not spam - seems valid, so should it be marked as correct=TRUE? I also don't follow the message for "incorrect=Given the kind of number an email contains, we can't say whether it'll be more or less likely to be spam". This seems like precisely what we can say from a segmented bar chart.
question("Which of the following interpretations of the plot **is not** valid?",
answer("Given that an email contains a small number, it is more likely to be
not spam"),
answer("Given that an email contains no number, it is more likely to be spam.", correct = TRUE),
answer("Given that an email contains a big number, it is more likely to be not spam.", correct = TRUE),
answer("Within both spam and not spam, the most common number is a small one.", message = "Wrong!"),
incorrect = "Given the kind of number an email contains, we can't say whether it'll be more or less likely to be spam.",
allow_retry = TRUE
)