1. The character set has already been defined in the question so regardless, nchar and nvarchar are out.
I see your point but I'm not really sure I agree with you. Just because he said it would be mixed letters and numbers doesn't mean that I won't need to process identifiers using letters and numbers from a different alphabet. My point is that he wasn't specific so a degree of ambiguity exists.
2. The average length isn't defined but it was a simple calculation. When building a database and constructing tables we rarely know what the contents of the table will be in advance. We therefore must assume they will be approximately equal if there is nothing to suggest the contrary.
He did say evenly distributed so I will retract my previous remark as that is pretty clear.
3. I take your point that varchar is more flexible and in future changes might make char less efficient, however at that point we're thinking outside the scope of the question. At the present time with the present facts, the choice is clear, however perhaps in a real setting you might choose differently given the purpose of the column
How is it that I'm thinking outside the scope of the question? My point is that he didn't clearly define the scope of what constituted "efficient" therefore any of the answers could be correct depending on your point of view. As you said, "perhaps in a real setting you might choose differently given the purpose of the column". Since he did specify that it was an "external identifier" I made the assumption that this identifier was the key to an external table. I don't want to have to use a TRIM function every time I refer to the column as that is decidedly NOT efficient and makes the queries less readable. It isn't that using a trim function is hard it just means that I have additional processing that must be performed with every row that is evalulated/returned. Not exactly my idea of efficient.
For me, efficiency is more about query performance and less about storage considerations. It's not that I discount the importance of storage considerations it's just that these considerations are less significant in this era of relatively cheap storage; however, they become more significant if I'm designing a multi-terabyte data warehouse for example instead of your run-of-the-mill 50 GB OLTP database. Consequently, the definition of "efficiency" may be vastly different.
"Beliefs" get in the way of learning.