Question

what causes a solution to a rational equation to be an extraneous solution?
if a solution results in a negative number when substituted into the denominator of the equation, the solution is extraneous.
when a solution is a fraction, the solution is extraneous.
if a solution results in zero when substituted into the denominator of the equation, the solution is extraneous.
when there is more than one solution, one of the solutions is extraneous.

Explanation:

An extraneous solution in a rational equation is a solution that does not satisfy the original equation, often because it makes the denominator zero (since division by zero is undefined). The first statement correctly defines this: if a solution makes the denominator zero when substituted, it's extraneous. The other statements are incorrect (a solution being a fraction, negative, or one of multiple solutions doesn't make it extraneous by those definitions).

Answer:

If a solution results in zero when substituted into the denominator of the equation, the solution is extraneous.