Right option is (d) Serum drug level
The explanation: When bioavailability measurement using pharmacokinetic processes are difficult, inaccurate, non-reproducible, acute pharmacological effect such as a change in ECG, EEG, pupil diameter, etc. is related to the time course of a given drug. Bioavailability can then be calculated by construction the pharmacologic effect-time curve as well as dose-response graphs.