Detailed option-wise explanation:

1. Alkaline phosphatase

• A hydrolase that removes phosphate groups from many substrates.

• Used in some diagnostic assays (e.g., ELISA), but not the classic choice for specific glucose detection in blood biosensors.

2. Glucose oxidase – correct

• Flavoprotein enzyme that specifically oxidizes β-D-glucose to gluconic acid while reducing oxygen to hydrogen peroxide.

• In most electrochemical blood glucose biosensors (glucometers), glucose oxidase is immobilized on the electrode; the produced hydrogen peroxide or electron transfer from the enzyme is measured as a current proportional to glucose concentration.

• High specificity and robustness make it the standard recognition element for glucose estimation.

3. Glycogen phosphorylase

• Enzyme in glycogen breakdown, catalyzing phosphorolysis of glycogen to glucose-1-phosphate in tissues like liver and muscle.

• Not used in glucose biosensors because it does not directly or specifically measure free blood glucose.

4. Glucokinase

• A hexokinase isoform that phosphorylates glucose to glucose-6-phosphate in liver and pancreas, functioning in glucose sensing and metabolism.

• While biologically important, it is not typically immobilized in commercial blood glucose biosensors.

Therefore, for estimation of glucose from a blood sample using biosensors, the enzyme used is glucose oxidase (option 2).