A low-pass filter with a roll-off rate of 60bB/decade is needed. The best combination is:
1 a 2pole filter followed by another 2-pole
2 two single-pole filters in series
3 a 2-pole filter followed by a 1-pole
4 none

 

Each pole contributes a roll-off rate of 20dB/dec.... so I would imagine you need a total or 3 poles....ie: choice 3