Answer:
[tex]\sqrt{gr}[/tex]
Explanation:
Let the radius of the loop be r. When the rider is upside-down, only gravitational acceleration g = 9.81 m/s2 and centripetal acceleration a acting in opposite direction to keep you balanced. Therefore
a = g
[tex]v^2/r = g[/tex]
[tex]v^2 = gr[/tex]
[tex]v = \sqrt{gr}[/tex]
So the minimum speed must be [tex]\sqrt{gr}[/tex] so the rider does not fall our of the car