Explanation:

Correct answer is (a).The given sentence is an example of a dangling infinitive. At first the sentence may seem correct but there is no one doing the ‘understanding’. ‘To understand English better’ is an action that should be linked to a doer  who should appear as a subject for this clause. The best way to correct this sentence is to introduce a subject at the beginning of the sentence. The corrected sentence should read – ‘For you to understand English better, a good dictionary is necessary.’ So the error is in the first part of the given sentence.