What is |3 - 4i|?

The modulus of $a+bi$ is the distance from the origin in the complex plane: $\sqrt{a^2+b^2}$. For $3-4i$ it is $\sqrt{3^2+(-4)^2}=\sqrt{9+16}=\sqrt{25}=5$, choice H. This is the familiar 3-4-5 right triangle. Choice J ($7$) adds the absolute values of the parts instead of using the Pythagorean theorem. Choice G ($1$) is $3-4$ with the sign dropped, and choice F ($-1$) is $3-4$. A modulus is a distance, so it can never be negative, which rules out choice F immediately.