Question on Pareto optimality problem
As denesp mentioned, your solution is incorrect. To check whether there is envy, you have to compare the two bundles from the point of view of the same agent. So in our case we have:


For agent A: the utility of his own bundle is $4 \alpha$ and the utility of the other bundle is $\alpha+5$. So he feels no envy as long as $4\alpha \geq \alpha+5$, which is equivalent to: $\alpha \geq 5/3$.
For agent B: the utility of his own bundle is 5 and the utility of the other bundle is 0. So he never feels envy.


This means that the correct answer is (c).