Could it not also be "This part is the shallowest,..."?
That would be, "Bagian ini yang paling dangkal/terdangkal..."
That is perfectly correct and means exactly the same thing. I think the translation that is marked as correct is probably a pedantic interpretation by a non-native English speaker.
I wrote "This is the most shallow part. The deepest part is there." But I'm pretty sure my answer should be accepted. Probably rejected because I refused to make a run-on sentence in English.
ada vs adalah?
I don't think that 'shallowest' is the correct form. It can be changed to MOST SHALLOW.
Shallowest is a more natural english usage
this is the most shallow part
The translated English is correct, except that it would usually consist of two sentences.
There are many ways to say this in English. Mine was not one deemed correct though it is.