Pure water has a ph of 7. the inside of a cell is also about ph 7. this might be so because buffers work best when the ph is close to 7. humans are largely made up of water by weight. most of the chemical reactions that occur in humans can proceed optimally at ph 7. ionic bonds cannot form at ph 7.

The pH of the blood is 7.4 and the pH of the human cells within is 6.8, that is, are both very near to neutral pH. The extreme values of pH, that is, either below or above 7.0 are generally regarded as not suitable for life.  

The majority of the species, involving humans, require to sustain pH within a fairly marginal range in order to survive. For example, the human blood requires to maintain its pH at about around 7.4. The buffer solutions that can resist the variations in pH are essential in sustaining stable hydrogen ion concentration in the biological frameworks .

the correct answer is d