Step 1: Idea:
Remember the word itself. To explode means to push a slice outward. The value you give is a fraction of the radius.
Step 2: Match each option with its own argument.
Border: wedgeprops. Colour: colors. Rotation: startangle. Offset of a slice: explode.
Step 3: Read what explode takes.
It takes a list with one value per slice. A value of 0.1 moves that slice by 10 percent of the radius.
Step 4: Choose the option that says this.
Option 3 states the radius fraction, so it is the answer.
Step 5: Test every option against the result.
Option 1: Adding a border is a different setting, so this does not describe explode.
Option 2: Colour has its own argument, so this does not describe explode.
Option 3: A fraction of the radius that moves a slice outward is the exact job of explode, so it fits.
Option 4: Rotating the pie is done by the start angle, so this does not describe explode.
Step 6: Conclusion.
Option 3 is correct.
\[ \boxed{3} \]