The only statement that must be true is that the function is increasing from left to right.
We are given that f is a one-to-one function and that f(3) = 2 and f(-4) = 8.
Since f is one-to-one, we know that the function has no repeating values. In other words, for any two distinct input values, f will output two distinct values.
We can use this information to eliminate options A, B, and D. These options all claim that f must have a value of 2 or 8 for some input. However, we know from the given information that f(3) = 2 and f(-4) = 8. This means that the function cannot have any other values of 2 or 8, since it is one-to-one.
The only option that remains is C, which states that the function must be increasing from left to right. This means that as the input value increases, the output value must also increase. We can confirm that this is true by noticing that f(-4) = 8 is greater than f(3) = 2. This means that the function is indeed increasing from left to right.
Therefore, the only statement that must be true is that the function is increasing from left to right.