We may compute the value (price) of any derivative of interest once we know its payoff function and have reasonably assumed a stochastic process for its underlying asset.
By Absence of Arbitrage, assuming no dividends are paid, for any $t \leq T$,
alternative notation[ref]Generalised in Björk, Tomas. Arbitrage Theory in Continuous Time, third edition. Appendix A, Definition A17.[/ref]: