The normal gain in length in a full term baby, at first 6 months of life is:

c. 15cm(Ref: Nelson's 20/e p 84-89, Ghai 8/e p 13-14)Lengthatbirth=50cmLengthat3months=60cm;So,gaininlengthin3months=60-50=10cmLengthat9months=70cm;So,gaininlengthin9months=70-50=20cmSo, gain in length in 6 months is between 10and20cm, so 15cm is the best answer.