Whichever way we choose to interpret it, the triangle 1 definitely does not cover the pixel and triangle 4 definitely does. If we take the interpretation that any pixel touched by a triangle, then both 2 and 3 are covered. If we take the center of the pixel as its "location", and compute whether that center is inside the triangle, we will only cover with triangle 3.
Note to students: we came back to this question and answered it in Lecture 2:
See slide 23 of Lecture 2.