THE PH OF THE CELL SAP OF OPUNTIA IF TESTEDEARLY EVENING WILL BE..........1] 4.5 2] 5.0 3] 6.9 4] 4.0

Option 2 is right.

During night, the organic acid content of the plant increases and pH of their cell sap decreases where as during the day, the organic acid content decreases and pH of their cell sap increases.

Since it is tested in the evening time, the acid content of the plant slowly start increases and the pH would slowly start decreasing.