Put-call parity is given by $C + Ke^{-r(T-t)} = P + S$.
The variables $C$, $P$ and $S$ are directly observable in the market place. $T-t$ follows by the contract specification.
The variable $r$ is the risk-free interest rate -- the theoretical rate of return of an investment with zero risk.
In theory that's all very simple. But in practice there is no one objective risk-free interest rate.
So in reality, how would you go about setting $r$? Why?
Answer
If your trades are collateralized/margined, you should use the rate paid on your collateral/margin.
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