To solve this, we need to find the amount of flour that corresponds to using 1 teaspoon of yeast, according to the given recipe ratio.
The original recipe states that for every 3/4 teaspoon of yeast, you need 1/2 cup of flour.
First, we can set up a proportion based on the yeast:
\[ \frac{3/4 \text{ teaspoon yeast}}{1/2 \text{ cup flour}} = \frac{1 \text{ teaspoon yeast}}{x \text{ cup flour}} \]
Now we cross-multiply to solve for \(x\):
\[ (3/4) \times x = (1) \times (1/2) \]
This simplifies to:
\[ (3/4)x = 1/2 \]
Now, we can solve for \(x\) by multiplying both sides by \(4/3\):
\[ x = (1/2) \times (4/3) = 4/6 = 2/3 \text{ cup} \]
So, if Oscar plans to use 1 teaspoon of yeast, he needs 2/3 cup of flour to maintain the same taste in the bread.
Therefore, the correct answer is: C. 2/3 cup.