Answer :

MD (M stands for 1000 and D for 500)
1500 = MD            as M = 1000  and D = 500
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For information:

I = 1          V = 5             VI = 6
IX = 9          X = 10         XI = 11
XL = 40        L = 50            LI = 51     LXII = 62

C = 100        CI = 101       
D = 500        DI = 501

MDC = 1600

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