My hypothesis is as follows: "Nadharia" comes from "nadhari", meaning something like discernment. It's an N- group word, meaning singular and plural are the same. "Tete" is a fully formed but not yet a fully riped grain. So, with a bit of imagination you can read it as a not yet fully riped theory, i.e. a hypothesis / hypotheses.
I am not really sure but I think this is just a compound noun and it would be translated as singular or plural depending how the English sentence is constructed (I guess if the Swahili noun here is belonging to Ma class nouns, then this could represent a collective noun for the Swahili paradigm. The noun nadharia is also translated as theory/theories and I think this is also a collective word for ideas/explanations with more acceptance than a simple hypothesis.)
Some sentence examples (Sorry if these are very long sentences but it is just to show uses for nadharia tete in a Swahili text):
1. Huenda ukafahamu kwamba usemi nadharia tete “wamaanisha uthibitisho usiotosha ambao watoa ufafanuzi uwezao kubadilika.”
As you may know, a hypothesis “implies insufficient evidence to provide more than a tentative explanation.”
2. Baada ya kusema hayo, ensaiklopidia hiyo ilitoa mambo manne ambayo yawezekana yalisababisha hali hiyo, lakini ikasema kwamba mambo hayo ni “baadhi ya zile nadharia tete nyingi.”
After saying this, the encyclopedia just quoted offered four possible explanations but said that these are “among the many hypotheses.”
From Glosbe: https://glosbe.com/sw/en/nadharia%20tete
The way the encyclopedia used the word is the way I have seen it used in English. It is simply the explanation one is putting forth for further research. One might favor one hypothesis, or one might investigate several hypotheses. It sounds as if either the Swahili does not have a separate plural or possibly the lesson's designers have not yet caught up with a typographical error of some kind here.
I think it is the same combination of words that make the singular and plural. Once there is a context it is possible to choose between them. For example, in the second sentence: nadharia tete nyingi would be the plural form, many hipotheses. Sometimes, we would have nadharia tete that means the singular, and sometimes the plural. I just insist this is that I think, and I would need to see if this noun is in the N-class to can affirm better this. Still I do not understand if the course do not accept the singular here, but I think it would be accepted in the future.