The 20 yard line
The 30 yard line.
35 yard line
College football kicks off at the 35-yard line.
1955 through 1887
40 yard line
The opposing 20 yard line
"40 yd line." Wrong - it is the 30 yard line now. HS still uses the 40 but college went to the 30 in 2007.
The team who the safety was against has to kick off to the team that scored the safety (free kick) from the 20 yard line. A free kick means that they can kick off using a tee, or they may punt the ball.
The kickoff is from the kicking team's 30 yard line making the kickoff travel 70 yards to reach the end zone.