Nice, easy question - though I'm sad that as of this time, 8% of respondends got it wrong.

There is an error in the explanation, though. You can not avoid the implicit conversion. You can only change it. Making the parameter varchar(100) instead of nvarchar(20) means that now the implicit conversion from an implicit conversion from varchar to nvvarchar during the scan, to an implicit conversion from nvarchar to varchar during the procedure call.

Or you can leave the parameter as is and add an explicit cast in the query. Again, not avoiding the conversion, but making it explicit.