The correct options are
B Incorrect= of, Correct= off
D Incorrect= of, Correct= about
The sentence talks about the likelihood of Soomro becoming the head of his department. 'About to become' indicates that it has already been decided that he should become the head of department. So, it is a probable answer. 'Off to become' also indicates that he is set to become the head of department. Hence, option C is another possible answer. The phrases 'for to become' and 'in to become' are grammatically incorrect in the context of the sentence.