Verified answer

For solutions C and D, note that [H3O+] = 10^-14/[OH-], which will then allow you to directly calculate the pH of every solution. You can then rate them from most acidic to most basic (low numbers are more acidic, of course).

The highest pH and the "most basic" will be the same. The "least acidic" and "least basic" will be the acid (or base) closest to a pH of 7.

Solution A has [H3O+] = 5.0×10-8 M pH = 7.3

Solution B has [H3O+] = 1.1×10-7 M pH = 6.9

Solution C has [OH-] = 2.0×10-8 M pOH = 7.7 so pH is 6.3

Solution D has [OH-] = 4.3×10-8 M pOH = 7.4 so pH is 6.6

Highest pH is lowest [H3O^+] so solution A

Least basic is lowest [OH^-] so solution C

Least acidic is lowest [H^+] so solution B

Most basic is highest pH so A