A European option is a contract in which the seller of the option grants the buyer the right, but not the obligation, to purchase from or sell to the seller the underlying asset at pre-specified price at maturity date. Herewith the buyer should pay out a premium for the value of flexibility that he was granted. Such premium as the compensation to the seller was provides in close form by Black and Scholes (1973) and Merton (1973). Even since then the option pricing methodology, or otherwise known as ¡§contingent claim analysis¡¨ has found its application in many prospects. Otherwise known as the real option analysis first induced by Myers (1977) and the structure form model of the credit risk analysis first induced by Merton (1974). In the thesis, we consider the application of the optional pricing methodology to the rationality and valuation of penalty in a contract and extent the penalty to the money back guarantee. In the former, we provide the general form solution to illustrate the both parties all hold the right to default the contract, and prove the existence of the optimal penalty is a policy to protect the disadvantaged minority such as to make the trade contract to be fair. In the latter, we prove the suitable way to evaluate that the consumer buy a good and long a MBG is the call option but the put by reviewing the final cash flow of the replicated strategy and the put-call parity at firstly, and then we find out the better way to grant the consumer to return the good to the vendor is penalty if the good is normal and the utility function of the consumer is concave. In sum, we integrate the penalty and in the MBG with the contingent claim analysis in this thesis, we find out we can use the uncomplicated model to explain the real world. Herewith we consider the option pricing model as another methodology to illustrate the social environment.