f[ϵ_,λ_,T_,c_]:=c ϵ+c^2 λ-T(-(1/2+c) Log[1/2+c]-(1/2-c)Log[1/2-c])

Manipulate[Plot[f[ϵ,λ,T,c],{c,-1/2,1/2},PlotRange->{-1/2,1/2}],{{ϵ,0},-1,1,Appearance->"Labeled"},{{λ,0},-1,1,Appearance->"Labeled"},{{T,0.5},0,1,Appearance->"Labeled"}]