The answer is C.) " I saw a fine pickerel wriggling in the sun. 'Uncle!' I cried, looking back in uncontrollable excitement, 'I've got a fish!' 'Not yet,' said my uncle."
The detail best shapes the theme that one should only discuss the outcome of a task when it is complete is this passage because as he looks back in the water his hook is empty and he lost his prize: the fish. So the lesson he learns is you should not boast of anything until the task is complete as the proverb universal "Never brag of your fish before you catch him.".