"This relationship is believed to make his mother happy."

Translation: Hubungan ini diyakini akan membuat ibunya bahagia.

[AdamZulkif]

The other option is more correct in my opinion. If you want to say "'akan' membuat ibunya bahagia" then it should say "believed 'will' make his mother happy"

[mapmanic]

I'm not feeling the 'akan' in the English gloss. Sounds current or habitual to me.

[Ms.Fixer]

Taking into account explanations from native speakers, I would suggest Duolingo to change the English translation as follows:

"It is assured that this relationship will (or shall) make his mother happy."

First off, "diyakini" is stronger than just "to believed", according to answers from native speakers as well as other Duo course-takers' search. "It is assured that..." may not be the best choice of words, so please feel free to give your alternative suggestions.

Second, as @AdamZulkif and @mapmanic pointed out, "akan" was not translated. This is a critical mistake because there is an important difference between "diyakini" and "dipastikan". As we've learned in a previous lesson, "pasti" also means "certain/definitely". However, "diyakini" is based on strong personal belief (i.e. subjective) whereas "dipastikan" is based on validated/confirmed facts/evidence (i.e. objective). That's why "diyakini" + "akan" (will/shall) is a good collocation. We shouldn't drop the nuance of "akan" from this sentence.

[Peachywaxcap]

I think of taking as confident.
How about, This relationship will surely make his mother happy.