Tom is interested in getting advice on what is the best outcome at the end of a six-year period for investing a sum of money in the following options, given a 9% per annum earning rate. What is the preferred option? 

a. Invest $8,000 as a lump sum today. 

b. Invest $5,000 at the end of each of the next six years. 

c. Invest a lump sum of $6,000 today and $1,000 at the end of each of the next six years.

 d. Invest $1,200 at the end of years one, three and five