a. 29% of 427 is 124 women buying books online.
b. We have no standard deviation (SD) to work with. The value 1.96 is associated with the 95% confidence interval since the sample size is quite large. If we assume a binomial distribution, we can estimate the SD=√427*0.29*0.71=9.38. In percentages this is a variation from 29% of about 2.2%, so we can quote 29±2.2% as the statistic.
Now, using 2.2% as the SD, we divide it by √427: 0.11% and apply the confidence coefficient value: 0.11*1.96=0.21% approx.
We use this to give us the margin of error: 29±0.21% as the mean and SD of the percentage of women who buy books online. We have 95% confidence that the number of women lies in this range.
c. We can therefore safely conclude that less than 40% of all women purchase books online, since 40% is much larger than the boundary 29.21% from the confidence interval.
d. We can also fairly safely conclude that at least 28.79% of women purchase books online, so that would cover at least 25%.