Answer
We can rank the experiments in order of the stopping potential, from largest to smallest:
$d \gt a = b \gt e \gt c$
Work Step by Step
We can find $K_{max}$ in each case:
(a) $K_{max} = E-\phi = 2.8~eV-2.0~eV = 0.8~eV$
(b) $K_{max} = E-\phi = 3.0~eV-2.2~eV = 0.8~eV$
(c) $K_{max} = E-\phi = 3.0~eV-2.8~eV = 0.2~eV$
(d) $K_{max} = E-\phi = 3.0~eV-2.0~eV = 1.0~eV$
(e) $K_{max} = E-\phi = 2.8~eV-2.4~eV = 0.4~eV$
Note that the stopping potential is proportional to the maximum kinetic energy.
We can rank the experiments in order of the stopping potential, from largest to smallest:
$d \gt a = b \gt e \gt c$
