Public Overridable Sub WriterOptionSetByIndex( _
   ByVal _eType As eMFWriterOption, _
   ByVal _nIndex As Integer, _
   ByRef _ppProps As IMFProps _
) 

public virtual void WriterOptionSetByIndex( 
   eMFWriterOption _eType,
   int _nIndex,
   out IMFProps _ppProps
)

public procedure WriterOptionSetByIndex( 
    _eType: eMFWriterOption;
    _nIndex: Integer;
   Out  _ppProps: IMFProps
); virtual; 

public: virtual void WriterOptionSetByIndex( 
   eMFWriterOption _eType,
   int _nIndex,
   [PARAMFLAG::Out] IMFProps* _ppProps
) 

Parameters

_eType

option's type (format, video or audio codec)

_nIndex

option's index

_ppProps

properties of the option (attributes for configuration)

Here is example of how set audio codec by selected index:

IMFProps props;
m_objWriter.WriterOptionSetByIndex(eMFWriterOption.eMFWO_AudioCodec, audiocodec.SelectedIndex, out props);
Marshal.ReleaseComObject(props); // release props if you don't need to use it

Reference

MFWriterClass Class
MFWriterClass Members