Answer
The pH of the solution will decrease.
Work Step by Step
Since $HB^+$ is an acid, it makes sense that it will raise the $[H_3O^+]$ of the solution, reducing the pH.
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As we discussed on the last exercise (17.14b), after the addition of $HB^+$, the equilibrium is shifted to the left side of the equation:
$B(aq) + H_2O(l) \lt -- \gt HB^+(aq) + OH^-(aq)$
Which implies a greater consumption of $OH^-$.
When $[OH^-]$ decreases, the hydronium ion concentration $[H_3O^+]$ increases, which will cause the decrease of the pH.
