A child is able to say sentence of 6 words at:

d. 5 years(Ref: Nelson's 20/e p 65-77, Ghai 8/e p 52-53)Mnemonic:Between ages of 2 and 5 years, number of words in a typical sentence equals the child's age (e.g. 2 by age 2 year, 3 by age 3 year)So, a child speaks 5 word sentences at 5 years; So, out of the given options 5 years is the best answer.