Colleen and Bill have just purchased a house for $650,000, with the seller holding a second mortgage of $100,000. They promise to pay the seller $100,000 plus all accrued interest 5 years from now. The seller offers them three interest options on the second mortgage: 

(a) Simple interest at 6% per annum 

(b) 5.5% interest compounded monthly 

(c) 5.25% interest compounded continuously

Which option is best? That is, which results in paying the least interest on the loan?