Which solution has the greatest number of hydrogen ions?

a) blood, ph = 7.2
b) lemon juice, ph = 2
c) soda, ph = 3.5
d) bleach, ph = 13.5

The Answer is option B.
pH is a scale for measuring either the solution is acidic or basic or neutral. In another way we can also say that pH is the measurement of concentration of hydrogen ions. 
when we see the relationship between the hydrogen ions and pH, we see they are inverse relationship means if pH is low, the solution has greater hydrogen ions. So as the pH value increases the amount of hydrogen ions decreases.
That is the reason lemon juice with pH 2 has greater number of hydrogen ions.