"Their help is very helpful."

Translation: Bantuan mereka sangat membantu.

[AlexErdman]

Is membantu an adjective in this sentence?

[WayangOrang]

'membantu' is a verb, the English translation is an adjective.
I think there is no other way to translate this with a verb in English.
Well, at least, I don't know which verb to use, and how to use it in English.
"Their help is helping"….this is weird, this doesn't work.

[GuruJulius]

How about: "Bantuan mereka sangat bermanfaat." It seems to me that the mem+bantu would demand an object (siapa) which is absent here. Does Bantuan mereka sangat membantu" sound normal to you as a native speaker?

[WayangOrang]

How about: "Bantuan mereka sangat bermanfaat."

I don't know if it's an accepted translation.

'bermanfaat' = useful, beneficial, advantageous

It seems to me that the mem+bantu would demand an object (siapa) which is absent here.

Yes, 'membantu' is a transitive verb.
With some transitive verbs, the object is optional (semitransitif).

Examples semitransitif (object is optional):
5a1) Tini menyapu. (S-P)
5a2) Tini menyapu rumah. (S-P-O)
5b) Rumah disapu oleh Tini.
6a1) Tini sedang membaca. (S-P)
6a2) Tini sedang membaca buku. (S-P-O)
6p) Buku sedang dibaca oleh Tini.

These are examples from this grammar topic:
Transitive & Intransitive verbs, Tips & notes, Addendum.
https://forum.duolingo.com/comment/29750515

'membantu' is semitransitif (object is optional).

[digitalpointer]

I'd say "Their help helps a lot." Sound dumb/repetitive, but not wrong.

[Bev425285]

Why not Bantuan mereka adalah sangat membantu

[DennisAndreas]

Why is "berguna" for the word helpful not accepted while it is part of the possible answers?

[Flanders1]

Why is "Pertolongan" marked incorrect when it is one of the options given?

[Wira_basa]

Please report it.

[Artemios5]

Bantuan mereka membantu sekali : marked wrong