By using Einstein photoelectric equation, if we consider kinetic energy of photo electrons on y-axis and frequency of incident radiation on x-axis. The slope of the graph will represent (the symbols have their usual meanings)

$h$

The correct answer is Option (2) → $h$

The Einstein photoelectric equation is -

$K_{max}=hv-\phi$ [$y=mx+c$]

where,

$K_{max}$ = Max. K.E. of photoelectron

h = Planck's Constant

v = frequency of incident radiation

$\phi$ = Work function