"Who is caught?"
Translation:Siapa yang ditangkap?
So, I wrote "Siapa ditangkap" and was counted wrong because I did not insert "yang."
I can understand that this may mean something closer to "Who is it that is caught," but I cannot understand why that is necessary.
Apa and siapa cannot occur as subject of a basic passive clause.
Your sentence 'Siapa ditangkap' is a basic passive clause, the same as :
'Tini ditangkap', 'Tono ditangkap', 'Dimas ditangkap'
'Siapa ditangkap', this sentence has 'siapa' as subject.
That's officially not allowed.
That's the reason they want you to use 'yang'.
By inserting 'yang', it becomes an identifying clause.
'yang ditangkap' becomes the subject of the sentence.
'siapa' becomes the verb/predicate.
Like the two sentences below:
'Siapa yang ditangkap?'
'Who was the one that was caught'
'Yang ditangkap siapa?'
'The one that was caught was who?'
If 'apa' & 'siapa' occur as part of the subject in a passive clause, then the same construction is used.
'Siapa yang ditangkap oleh polisi?'
'Siapa yang dibawa oleh Tini?'
'Apa yang dimakan oleh Tini?'
'Makanan apa yang dihabiskan oleh Tini?'
'Sepatu apa yang dibeli oleh Tini?
The above sentences cannot work without 'yang'.
'yang' does not only change the structure, but gives extra emphasis to the word (noun phrase) preceding it.
In the above sentences, it's the extra emphasis on what/who, apa/siapa.