Let's analyze each event
EVENT 1:
Since there are only black and blue pens in box A, it is a certanity that if you pick one at random, you'll choose a black or blue pen. This way,

EVENT 2:
Since there are no green pens on box B, you cannot pick a green pen out. This way,

EVENT 3:
Out of the 20 pens in box B, 5 are black. This way, the probability of choosing a black pen from box B is:

EVENT 4:
Out of the 8 pens in box B, 3 are black. This way, the probability of choosing a black pen from box B is:

This way, the list of events sorted from most likely to lest likely is
• Event 1
,
• Event 4
,
• Event 3
,
• Event 2