One of the following values for $a$ makes the expression $\dfrac{2a+5}{a^2+1}$ undefined. Which one?

$-3$

$-\dfrac{5}{2}$

$-1$

$0$

$i$

Approach

The expression is undefined when the denominator is equal to 0.

$$ a^2+1 \neq 0 $$ $$ a^2 \neq -1 $$ $$ a \neq \pm\sqrt{-1} $$ $$ a \neq \pm i $$