Answer:
(a) Randy's opportunity cost of mowing lawns = 

 = 3 
Therefore, 3 trees have to be foregone for mowing 1 lawn.
Randy's opportunity cost of pruning trees = 

 = 0.33 
Therefore, 0.33 lawns have to be foregone for pruning 1 tree.
(b) Frank's opportunity cost of mowing lawns = 

 = 2 
Therefore, 2 trees have to be foregone for mowing 1 lawn.
 Frank's opportunity cost of pruning trees = 

 = 0.50 
Therefore, 0.50 lawns have to be foregone for pruning 1 tree.
(c) Frank has a comparative advantage in mowing lawns and Randy has a comparative advantage in pruning trees. 
(d) Randy has a absolute advantage in both, mowing lawns and pruning trees.