This question is testing your familiarity with the hierarchy for the interface. A few notes on taking multiple choice tests.
When looking for the correct answer search the command in google and add site:juniper.net this should give you the documentation page in the results as posted above so you can verify the right answer.
set interface fe-0/0/0 mtu site:juniper.net
During a test try to eliminate answers you know are wrong. So you correctly identified that two answers would be the same thus you know both cannot be correct because only one answer is called for here. But be careful as some questions specify choose 2 or 3 correct answers. With those eliminated you have now a 50/50 chance of guessing correctly.
What helps to prepare for questions like this is hands on use of Junos devices. You will remember these heirarchy questions better the more you actually type these commands into devices. So spend as much time as you can configuring on the cli not just reading for your preparations.
Best of luck on the exam.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home