Su Mo first eliminated Second Princess from his selection. This wasn't because she was a lady but because she had made a good impression on him, a far better impression than the First Prince and Zhu Tong gave im.
As such, Su Mo was choosing between the First Prince, Third Prince, Zhu Tong and Xu Feng.
The person that he was most itching to challenge was the First Prince. After all, Su Mo had wanted to challenge the First Prince at the birthday banquet but the First Prince had rejected him by saying he didn't have the qualifications to do so.
Su Mo was not going to rush to take action. Instead, he would observe the strength of the others. Although his current strength was definitely stronger than the average Rank 9 Martial Honorable's, he would not underestimate his opponents.