Four years ago the average age of a family of four members was 18 years. During this period a new baby was born. If at present the average age of the family is the same as it was four years ago, then the present age of the baby is (yrs.)

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.