TLDR: The common noun is "sandwich".
A common noun is a person, place, or thing that isn't being described specifically. A proper noun is a person, place, or thing that is being described specifically. For example:
A house (CN), versus The White House (PN)
A Meatball Marinara (PN) versus a footlong sub (CN)
New York City (PN) versus The Big Apple (PN) versus a city (CN)
Proper nouns can typically be identified by being capitalized, however this may not always be the case! However, in this example, it is.
Start by looking for nouns - this sentence contains two, Carlene and sandwich. A sandwich is a common noun because it is generic, whereas Carlene is being specific - we're talking about a particular girl who happens to like sandwiches. Therefore, "sandwich" was the common noun in this sentence.
Hope this helps!
