That's a question Google has heard before. Google says the answer is "As". So that's probably the answer your quiz is looking for. But I can tell you, in terms of Plain English, many words can fit just fine here. For example, another suitable word would be "Since". If I understand your post correctly, you are saying you think it might be "Once" or "If"... To these I would say, both would be understandable, but quite a slang / low-class form of grammar. To improve the grammar, if using "Once" or "If", you would also say "will bode well". So I return to "As" and "Since" as examples of ideal ways to fill that gap, and Google confirms "As" is the answer your quiz is expecting.

"If" Si estos cultivos pueden prosperar con tan poca agua, eso es un buen augurio para los agricultores africanos.

No preposition makes the sentence good or natural. Good are: These crops thrive with very little water. That bodes well for African farmers. That these crops can thrive with so little water bodes well for African farmers. Though spoken English is often disjointed, the phrase ‘bodes well’ is higher register and requires a different structure.

As, Since, Given

The missing word is likely "if": "If these crops can thrive with so little water, that bodes well for African farmers." Your attempt is right on track! "If" works best because it introduces a condition.