'Occupational mobility': the ability to move from one occupation (job) to another.
'The child's vocation...' Vocation means 'calling', which means: a profession, a job, an occupation.
'.... would develop along familial lines': in 17th century Enlish society, the child would normally take up the same occupation as the parents.
B is correct. 'Divergence from' means: go in a different direction from' .
C is not correct. Preparation for the career of one's parents would not necessarily be difficult. The assumption would be that preparation for a career that is already in the family would be easier than preparation for a different career; and the society's expectation that the child would follow in the career of the parents would likely disprove the child's choosing an occupation that differed from that of the parents.