"His last name is Wang."
姓 is most commonly used as a verb (to be surnamed). This is not typically done in English, so I understand your confusion, but I think it's always important to remember when learning a different language that you can't always translate word for word and retain the meaning due to cultural differences.
You can think of the verb 姓 as similar to German “heißen”, only that it specifically means “to be called by family (last) name”. 姓 can also be used as a noun “last name”, but it’s not particularly natural to use it as such in sentences like “what is your last name/my last name is”.
To ask someone in German what their name is, you would say/write "Wie heisst du?" = "What's your name?"
When someone/something is yet to be named, and the question is "What's he/she going to be called?" or "What's his/her name going to be?", you would write/say something like "Was sollt er/sie heissen?".
"Ich heisse The Flash" = "My name is The Flash" :D
There are so many errors in this course and littered with inconsistencies. Somebody should at least think about adding similar translations that are still valid, especially non-formal phrases.
His last name " IS " Wang
I used "ta xing shi Wang", which is correct as you asked for the word "is". Yet, your accepted answer is "ta xing wang" meaning "his last name Wang", ask yourselves if that makes any sense!!