Correct : a

The correct answer is Option A — Oxygenated hemoglobin absorbs less infrared light (900 nm) compared to deoxygenated hemoglobin.
Pulse oximetry works on a beautifully simple principle — oxygenated hemoglobin (HbO₂) and deoxygenated hemoglobin (Hb) absorb light differently at two specific wavelengths, and by comparing those absorption differences, we can calculate blood oxygen saturation (SpO₂).
The two wavelengths used are red light (~660 nm) and infrared light (~940 nm). Here''s what happens at each:
At red/visible wavelengths (~600-660 nm): Oxygenated hemoglobin absorbs more red light than deoxygenated hemoglobin. This is actually why arterial blood looks bright red — HbO₂ strongly absorbs red light. So Option B is false and Option C is true in reality, but let''s confirm against what the question is asking.
At infrared wavelengths (~900-940 nm): Deoxygenated hemoglobin absorbs more infrared light than oxygenated hemoglobin. So oxygenated hemoglobin absorbs less infrared — which is exactly what Option A states. This makes Option A correct and Option D incorrect.
A quick way to remember this — think of it as a crossover. At red wavelengths, HbO₂ wins (absorbs more). At infrared wavelengths, Hb wins (absorbs more). The two absorption curves cross near the isobestic point (~805 nm), where both forms absorb equally.
The pulse oximeter exploits this crossover by shining both wavelengths through the finger or earlobe and measuring the ratio of pulsatile absorption changes. That ratio is then mapped to SpO₂ using a calibration curve.