The correct option is A Wife
The mapping of the 4 statements can be shown as follows:
D (mother of A, so female)
B (son of A, so male)
D (father of B, so male)
C (wife of B, so C is female)
D (mother-in-law of C, so female)
B-C is a couple but their sex is unknown
A (daughter of C, so female)
D (sex unknown)
B (daughter-in-law of D, so female)
C (B's husband and D's son)
A (grand-daughter of D, so female)
In order to find out who is always speaking the truth, you need to think of a couple of things. The first one is that the relationship between B and C has to be clearly established, otherwise the answer to the second question in this set would become 'can't be determined' - an option that does not exist in this question.
If you were to use the relationship grids drawn above, it is clear that only B's or D's statements could both be true as they are the only ones defining a clear relationship between B and C. By a similar logic we need information about the relationships of each of the 4 people, from the statements of the person who speaks both statements true in order to test the truth of the options in the third question in the set. If we look at the statements of B, it is clear that although we do get a relationship between B, C and D in this case, we are not able to place A. Thus, we cannot assume both statements of B to be true.
This can only mean that both the statements of D must be true, as D's statements clearly identify the relationships between A, B, C and D.
B is the wife of C. Option (c) is correct.
DI and LR