One of Mr. Horton,his wife,their son,and Mr. Horton's mother is a doctor and another is a lawyer.

a)If the doctor is younger than the lawyer, then the doctor and the lawyer are not blood relatives.
b)If the doctor is a woman, then the doctor and the lawyer are blood relatives.
c)If the lawyer is a man, then the doctor is a man.

Whose occupation you know?

the question is wrong....if by any chance its correct the data is insufficient

Atleast one of them is doctor.
Statement a rules out son as he is a blood relative to all of them.
Statement b and c together rule out the possibility of the doctor being a woman

The only person whose occupation can be doctor, is Mr. Horton (which is not ruled out by any of the statements).

Mr. Horton's mother is a doctor

Father is Doctor
Son is lawyer

Mr.Horton-Doctor

According to the question

C) Says that there are only two chances that is
doctor and lawyer should be men or doctor and lawyer should be women.
B) There are only two women who are not blood relatives, so there are no chances of a women being a doctor.
A)
Remaining two men are blood relatives,  If the doctor is younger then the lawyer then they are not blood relatives, which is incorrect, therefore doctor is elder among them and who is Mr. Horton.

Son is lawyer
He (Mr.Horton) is Doctor

He is doctor
Wife is Lawyer

These are the two possibilities for the question.