Hello.
according to the list prepared by HSC authority for a standard conversion of marks in grades we can have an idea .
This chart is given below:

• 91-100. : A1
• 81-90.    : A2
• 71-80.    : B1
• 61-70.   :. B2
• 51-60.   :. C1
• 41-50.   : C2
• 33-40.   : D
• 21-32.   : E1
• 20 and below : E2

So with the reference of above chart your marks lie between 51-60 so your grade should be C1.
Best of luck.