Answer
(b) 0.47 s
Work Step by Step
The seed is launched at 90 degrees, so it will go straight up. Assume up to be positive.
The highest possible speed is $u = 4.6 m/s$. At the highest point, the velocity $v = 0$.
Using $ v = u + at$ (and using $a = -g$), we get $0 = 4.6 - 9.8t \implies t = 0.47 s$.
