Ok - I see what you're doing now. You want to find the common intersection of elements over all the columns.
> # making some fake data
> dat <- data.frame(a = letters[1:5], b = letters[2:6], c = letters[3:7])
a b c
1 a b c
2 b c d
3 c d e
4 d e f
5 e f g
> # for a single pair...
You'll need to be more specific. Are you looking to do this on a per column basis? Do you want to ignore the columns and treat all if the data combined? Maybe you're interested in unique rows? You didn't provide enough info in your post to know for sure.
This wasn't really a TIL but it's also RStudio specific. But there are a few examples of some helpful snippets you could include in RStudio to make your life easier - I'm too lazy to list them out so check out the blog posting https://jozefhajnala.gitlab.io/r/r906-rstudio-snippets/
I think C would be the typical answer if not for the word "approximate". That alone makes me think they want them to use 2 instead of the slightly more exact 1.96 when constructing the interval and in using 2 that does lead to (D).