The correct answer is (b) 2.
Four years ago, the total age of the family was $4 \times 18 = 72$ years. The present total age of the family is still 72 years. If the baby is $x$ years old, then the present total age of the family is $4 \times 18 + x$. Therefore, $4 \times 18 + x = 72$. Solving for $x$, we get $x = 2$.
Option (a) is incorrect because 1 year is not enough time for a baby to grow to the point where it would significantly affect the average age of the family. Option (c) is incorrect because 3 years is not enough time for a baby to grow to the point where it would significantly affect the average age of the family. Option (d) is incorrect because 4 years is the age of the baby four years ago, not the present age of the baby.