PASSAGE is about English Grammar.Look at the LINE: Indeed, to acquire the knowledge of grammar is not an easy task.From the above line, it can be concluded that ACQUISITION of grammar is not easy.The passage explains about the acquisition of English Grammar.It is said that: It should be learnt as a whole.It should be learnt in continuity.No part of it can be left unacquired.It can't be learnt in PARTS. Thus the correct answer is: "is not an easy task".