Question

For wch gas y=cp/cv will be maximum

Answer 1

γ is depends upon degree of freedom . E.g., γ = 1 + 2/f , here f is degree of freedom. Means higher the value of degree of freedom, lower will be value of γ.
As you know ,
For monoatomic gases , degree of freedom, f = 3 {all translational }
so, γ = 1 + 2/f = 1 + 2/3 = 5/3 = 1.67

For diatomic gases, degree of freedom , f = 5 {three translational , 2 rotational}
So, γ = 1 + 2/f = 1 + 2/5 = 1.40

For triatomic gases, degree of freedom , f = 7 {three translational, two vibrational and two rotational }
so, γ = 1 + 2/f = 1 + 2/7 = 1.30

What you see?, γ value for monoatomic gas is highest .
Hence , answer is monoatomic gases.