Answer
Tc-99 > Y-90 > In-111 > I-131
Work Step by Step
Given Y-90 half life is 64.1 hours
Tc-99 half life is 6.02 hours
In-111 half life is 2.8 days
I-131 half life is 8 days.
The nuclides having short half lives are most active.
So the order of most active to least active is:
Tc-99 > Y-90 > In-111 > I-131
