My mother was a bread winner of my family because at that time my farther ... working
A just began
B had just begun
The key is B but I don’t know somehow I think A is correct
I mean: ‘mom was a breadwinner’ happened before ‘farther ... working’ - 2 actions happened successively - so I think ... should be Simple Past Tense
The reason why i exclude option B is that I think using past perfect tense for ‘working’ means ‘working’ had happened for a while before ‘mother was a breadwinner’- seems a bit different from the context
Sorry for my bad english explanation

Question

My mother was a bread winner of my family because at that time my farther ... working
A just began
B had just begun
The key is B but I don’t know somehow I think A is correct
I mean: ‘mom was a breadwinner’ happened before ‘farther ... working’ -> 2 actions happened successively -> so I think ... should be Simple Past Tense
The reason why i exclude option B is that I think using past perfect tense for ‘working’ means ‘working’ had happened for a while before ‘mother was a breadwinner’-> seems a bit different from the context
Sorry for my bad english explanation

Answer

B would be correct, Liz,

I see your thinking  here, but i think about it like this:

The mother was working before the father, but she also continued working AFTER the father had started working. So, when we look at the time frame, we can still see that the POINT IN TIME that the father started working was before the "mother working period "  was finished (if it did finish).

Past Perfect is used when there are 2 past actions to show  which is MORE  past and in this case the father STARTING  work was before the mother's working period.

Sorry for the convoluted answer; does that make it a little clearer, Liz?