Isotope A has a half-life of seconds, and isotope B has a half-life of millions of years. Which isotope is more radioactive?
Well, if you really stare at the word "radioactive", you may notice it contains the fragment "active". So, more radioactive means faster half-life. In five minutes, isotope A will fragment many times, and isotope B will not at all fragment.
By inspection, isotope A is more radioactive.