Arectangle is formed by 2 congruent right triangles. the area of each triangle is 6 in^2. if each side of the rectangle is a whole number of inches, which of these could not be its perimeter .

How sad.  You were going along so nicely there, with a fascinating problem
that would be fun to work on, and then you suddenly fell off over the edge.

"Which of these ..." always means "Here's a list of choices.  Pick out the
correct answer from the list."  So we know that there was a list of choices
right there, where you copied the question from.  But when you finished
copying the question and reached the list of choices, you stopped there,
never copied the list, and went away to do something else instead.

Without that list, there's no way for us to answer the question "Which of these...".