You need to ensure that when the spreadsheet is rendered by Excel Web Services, the custom calculations are performed

You have a ShafePoint Server 2013 server farm that has Excel Web Services deployed.
A user has a Microsoft Excel spreadsheet that uses a dynamic-link library (DLL) to perform custom
calculations.
The user plans to upload the spreadsheet to the farm.
You need to ensure that when the spreadsheet is rendered by Excel Web Services, the custom
calculations are performed.
What should you add?

A.
A new trust relationship

B.
The DLL to the global assembly cache

C.
A trusted data provider

D.
A user-defined function assembly

Explanation:
Excel Services User-Defined Functions (UDFs)
Strong Name Dependencies
In some cases, a user-defined function (UDF) assembly depends on other assemblies that are
deployed with it. These dependent DLLs load successfully if they are in the global assembly cache, or
if they are located in the same folder as the UDF assembly.
Excel Services Best Practices