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C is corrent. The profit on a sale in the ordinary course of business is considered to be realized at the time of the sale unless it is uncertain whether the sales price will be collected. The Board concluded that use of the installment method of accounting is not acceptable unless this uncertainty exists.
A is incorrect.   The profit on a sale in the ordinary course of business is considered to be realized at the time of the sale unless it is uncertain whether the sales price will be collected. The Board concluded that use of the installment method of accounting is not acceptable unless this uncertainty exists.  
B is incorrect.  The profit on a sale in the ordinary course of business is considered to be realized at the time of the sale unless it is uncertain whether the sales price will be collected. The Board concluded that use of the installment method of accounting is not acceptable unless this uncertainty exists.  
D is incorrect.   The profit on a sale in the ordinary course of business is considered to be realized at the time of the sale unless it is uncertain whether the sales price will be collected. The Board concluded that use of the installment method of accounting is not acceptable unless this uncertainty exists.