(D) 7.0 to 7.8
Our body works within the pH range of 7.0 to 7.8.
Source: Acids, Bases and Salts, Section 2.3.1
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The passage explicitly states: "Our body works within the pH range of 7.0 to 7.8." This is a direct fact to memorise. Note that this range is slightly alkaline (above 7), which reflects the slightly basic nature of blood and body fluids. Options A and B are too acidic; option D is too alkaline.