Which one of the following is NOT a copolymer?

Stability of $$\alpha$$-Helix structure of proteins depends upon

The formula of the purple colour formed in Laissaigne's test for sulphur using sodium nitroprusside is :

A 2.0 g sample containing MnO₂ is treated with HCl liberating Cl₂. The Cl₂ gas is passed into a solution of KI and 60.0 mL of 0.1 M Na₂S₂O₃ is required to titrate the liberated iodine. The percentage of MnO₂ in the sample is _____________. (Nearest integer)

[Atomic masses (in u) Mn = 55; Cl = 35.5; O = 16, I = 127, Na = 23, K = 39, S = 32]