As the [ h3o+] in a solution decreases , the [ oh -]
a) increases and ph increases
b) increases and the ph decreases
c) decreases and the ph increases

 As the [H3O+]  in  a solution decrease, the [OH-]  increase and Ph increase ( answer A)

explanation

[H3O +]      are  released  by an acid while [OH-]    are released  by base.

The Ph of  an  acid is below 7  while that of a base is above 7.

as [H3O+] is decreased in solution  [OH-]  is increased   which make the solution more basic hence the PH is  increrased.

C) decreases and the pH increases.

Justification:

1) [H₃O⁺] and [OH⁻] in a solution are inversely related as per this equation:

[H₃O] [OH⁻] = 1 × 10⁻¹⁴

That means that if one increases the other has to decrease (to keep the product constant equal to 1 × 10⁻¹⁴).

2) The pH is defined as:

pH = log { 1 / [H₃O⁺] }

As [H₃O⁺] decreases, 1 / [H₃O⁺] increases, so being the logarithmic function a growing function, pH will increase.

That, explains why as [H₃O⁺] decreases the pH increases.

So, the lower the [H₃O⁺], the higher the pH, and the less acidic (more basic) the solution.