Fetal hemoglobin has a higher affinity for oxygen due to
Make it very clear in mind that the higher affinity of fetal hemoglobin for oxygen is due to the low affinity of HbF for 2,3 - DPG (not due to decreased 2, 3-DPG concentration). HbF does not combine to 2, 3-DPG as avidly as adult hemoglobin.
"Fetal hemoglobin is left - shifted because it has a low affinity for DPG". — Principles of medical physiology
"It is worth noting that DPG does not combine with fetal hemoglobin. That is perhaps one reason why HbF has a greater affinity for oxygen than adult hemoglobin (HbF)". — Understanding of medical physiology
"Fetal hemoglobin binds less 2-3 DPG it has consequently a greater affinity for O2". _ Pediatric physiology
All other textbooks have mentioned the same thing.
So, the high affinity of HbF to O2 is due to the low affinity of HbF for 2, 3 DPG (and not due to decreased 2, 3-DPG concentration).
However, amongst the given options, option a is the best answer here because, although there is no decrease in concentration of 2, 3-DPG, because of low affinity for 2, 3-DPG, HbF behaves as if 2, 3-DPG levels are low.
"Fetal hemoglobin has considerably less affinity for 2, 3-DPG than does adult hemoglobin. In human blood, the affinity of fetal hemoglobin for 2, 3-DPG is only about 40% that of adult hemoglobin. This makes fetal hemoglobin behave as if 2, 3-DPG levels are low; therefore, the oxygen dissociation curve is shifted to the left". — Clinical physiology in childhood
Low affinity for 2, 3 DPG
Reduced pH
Increased release of carbon dioxide
Oxygen dissociation curve is shifted to right