I have a question! I tried working out the problem but I got it wrong.

I have a question! I tried working out the problem but I got it wrong.  There's a solution attached. my question is why for the intertia of the rod(length = L), did they use 1/12 mL^2 instead of 1/12 m (L/2)^2. I thought the L is supposed to be the radius?