The sex of a newborn baby is determined by the chromosomes inherited from both parents. However, the primary determinant of the baby`s sex is the sex chromosome inherited from the father. This is because males have both X and Y chromosomes, while females have two X chromosomes.

Option 1: The mother does contribute a chromosome in determining the baby`s sex, as she always contributes an X chromosome. However, the baby`s sex is ultimately determined by the presence of a Y chromosome from the father.

Option 3: The mother`s mother (maternal grandmother) does not directly contribute to determining the baby`s sex. The sex chromosomes inherited by the mother are passed down randomly to her offspring.

Option 4: The father`s father (paternal grandfather) also does not directly determine the baby`s sex. The sex chromosomes inherited by the father are randomly passed down to his offspring.

Therefore, option 2 is correct because the sex of a newborn baby is primarily determined by the chromosome inherited from the father.