I doubt they'd ask this in the actual HSC, but deregulation just means the removal of government regulation or controls over the labour market, which then allows for a decentralised wage determination system. Decentralisation is where wages are determined by market forces and are negotiated between the employer and employee, rather than being determined by the government.
The two overlap, but basically, deregulation facilitates for decentralisation. So deregulation is the action that the government takes. Decentralisation describes the type of wage determination.
Hope that doesn't confuse you lol.