Which is best one time investment plan in India to get maximum return in long term of 15 years?

My sister is 35 years old. She wants to make a one time investment in such a way that she gets Approximately Rs 15000/- per month after she is 50. She does not want risk cover. Should she go for some pension plan? If yes which pension plan is best. Where other than pension plans can she invest where the returns are tax free. Should she go for unit linked plans or Mutual fund? How much will she need to invest? Please guide