can you write down all equations with explanations

Eight hundred (800) high school freshmen were randomly selected for a national survey. Among survey participants, the mean grade-point average (GPA) was 2.6, and the standard deviation was 0.4. What is the margin of error, assuming a 95% confidence level? (Show all your works, if you just only indicate correct answer, you will get only 5 points) (a) 0.04 , (b) 0.025 , (c) 1.960 , (d) 0.027 , (e) None of the these values