Hi,
whenever a sentence starts with verb+ing modifier, the subject should be present just after the comma. In A the comma should be followed by the person/committee. But, it is followed by 'the question'. So, it is incorrect.
Example: Sitting on the wall, a ball hit me (Incorrect)
Sitting on the wall, I was hit by a ball (Correct)

Problem with Option A.

a)In assessing the problems faced by rural migrant workers, the question of whether they are better off materially than the urban working poor is irrelevant.

Just read the first part of sentence before comma. ""
In assessing the problems faced by rural migrant workers"
The immediate question is who is accessing? This is a modifier clause.

In the second option B
b)In an assessment of the problems faced by rural migrant workers, the question of whether they are better off materially than the urban working poor is irrelevant.

First part is a phase.

I totally agree with Frankenstein on the same......

verb + ing modifier has a missing subject here in the second clause in A.The question is not accessing.Hence A is not correct.