A member of Girls' Generation did not recognize Son Seok Koo at all.
In the JTBC entertainment program 'Soshi Tamtam', which was aired on July 19th, Girls' Generation's participation in the extreme three missions was revealed.
The third mission was a 'memory game' to memorize pictures of 10 people who appeared while riding an alpine coaster on a monorail.
Hyoyeon, who started first, was worried, saying, "The actors will have an advantage. I can barely memorize the members' names." Yuri joked, "Hyoyeon barely memorizes her home address."
The 10 people in the photo were Kang Ho Dong, Kim Young Cheol, Son Seok Koo, Lee Hyori, Code Kunst, Jo Jung Suk, Doo-li, Jun Hyun Moo, Lee Yong Jin, and Eugene in that order.
Hyoyeon memorized only Kang Ho Dong and Kim Young Cheol, and when the picture of Son Seok Koo appeared, she said to the staff, "Who is after Kim Young Cheol? I really don't know. Just tell me once."
Hyoyeon, who was getting the correct answer at the destination, immediately failed by saying, "I don't know this person at all" at Son Seok Koo's turn.
Tiffany also said, "I'm sorry" without recognizing Son Seok Koo and Lee Yong Jin.
Source(1)