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A is corrent. ASC Topic 470 states that the debtor records a gain at the date of a restructure involving only a modification of terms when the prerestructure carrying amount exceeds the total future cash flows per the modification. The gain recognized is the difference between the prerestructure carrying amount and the future cash flows.
B is incorrect because no gain or loss is recorded if the carrying amount is less than total future cash flows.
C is incorrect.   ASC Topic 470 states that the debtor records a gain at the date of a restructure involving only a modification of terms when the prerestructure carrying amount exceeds the total future cash flows per the modification. The gain recognized is the difference between the prerestructure carrying amount and the future cash flows.  
D is incorrect.   ASC Topic 470 states that the debtor records a gain at the date of a restructure involving only a modification of terms when the prerestructure carrying amount exceeds the total future cash flows per the modification. The gain recognized is the difference between the prerestructure carrying amount and the future cash flows.