Doubt regarding Knowledge Check 12.2

Knowledge Check 12.2 asks which command will show the parent process IDs (PPIDs) for all the processes on the system. This question gives four possible options and marks only option 4 as the correct one:
1) ps aux
2) ps elf
3) ps-aux
4) ps -elf

I would like to know why multiple options are not possible if option 2 (ps elf) also shows the PPID of all the running processes. Why only option 4) is the correct answer?

Thanks