One should (1)/ look after (2)/ their parents.(3)/No error (4).

Answer

3

Explanation

one.s will replace their because-
one.s Is used as a Possessive Pronoun for describ- ing something thatbelongs to an individual.
Look at the examples given below:
One doesn.t like to spend too much time on one.s looks.
one should always look one.s best.