The correct options are
A b
C a
The option that has an error, is the correct option.
This sentence has two possible errors.
Option B: If we consider 'each of the boys' as a correct phrase, then in relation to that, the error lies in part (b): 'were to blame', where 'were' is the error.
'Each' refers to individual members of a group, and stresses on each person.
So, 'each' is a singular pronoun. 'Were' is a plural verb; the verb must agree with the subject in terms of person and number. A singular subject (each) will take a singular verb (was).
The correction is 'was to blame'.
Hence option B is the correct option with the error.
Option A: When we consider 'were to blame' as the correct phrase, 'each of the boys' has an error.
The correct would be: 'Both the boys/Both boys'
Hence option A is also correct with an error.
Part (c) does not have any error in it. It is grammatically correct and complete in meaning.
Hence option C is not the right answer.
Option D is incorrect because the sentence has two possible errors (a) or (b).