If the pk values for phosphoric acid are 2.15, 6.82 and 12.38, would predominate at ph 5 while would predominate at ph 10.

Phosphoric acid have three protons which is ionized in different . Thats why it is called triprotic acid. The protonisation of phosphoric acid in different can be shown as-→. This is the first ionization. The second and third ionizations are shown as-→+ and →   + P

=5 is near to first equivalence point where = ( + =(2.15+6.82)=4.6. At this , will be present.

At =10, which is near to second equivalence point= (6.82+12.38)=9.7. At  =10, will be present.

i believe the answer is true.

the answer your looking for is c).