Plot[{2/3, x, x^2, x^3}, {x, 0, 1},
PlotStyle -> {{Dashed, Black}, {Thick, Orange}, {Thick, Red}, {Thick,
Blue}}, AspectRatio -> 1/1, PlotRange -> {{0, 1}, {1/3, 1}},
FrameStyle -> Directive[Black, 16],
PlotLegends ->
Placed[LineLegend[{None, "No Protocol", "Repetition Protocol",
"Feedback Protocol"},
LegendFunction -> (Framed[#, FrameMargins -> 0,
Background -> White] &)], {Center, Top}], Frame -> True,
FrameLabel -> {{Style[#, Black, Bold, 16] &@("F"),
None}, {Style[#, Black, Bold,
16] &@("\!\(\*SubscriptBox[\(p\), \(1\)]\)"), None}} ]