Answer:
It really depends on what you are making. But a bread, generally, Yeast, flour, and the liquid, such as water.
Step-by-step explanation:
Yeast: Main and most important makes the bread rise and become bread.
Flour and Water: When the water, flour and yeast meet the gluten forms and stretches.
Water: Activates and dissolves the yeast so it is basically the most important ingredient other than the yeast. It also makes it sticky and stretchy.
The others that are useful but not the three are fat, sweetener, and egg.