Explanation:

Correct Answer is (d).The error lies in the given part in bold in the sentence as difficulty is noun whereas the adjective 'difficult' should have come here in its place. 
However, in option A position of 'farily' is wrong as per the grammar rules. Therefore, option A is eliminated. 
Among the available options, option D seems the most suitable replacement for the phrase given in bold.
The correct sentence would be:
Understanding the whole lesson at one was rather a difficult job for him but somehow he managed to do it.
It makes option D the correct choice among the given options.