Resistance only depends on the geometry and identity of the conductive material. Rearrange your equation, so that only geometry and identity properties are on one side, and the electrical operating conditions are on the other side. Recall the definition of resistance and recognize where you find it within this equation. This is how you would solve it, if the equation you presented were valid for this problem. It turns out it isn't valid for this problem and I'll explain what else comes in to play that you need to consider.
The formula you gave is built for the round trip voltage drop, hence the 2 in the numerator because current travels the full round trip in either a DC circuit, or a single phase AC circuit. For 3-phase, 2 gets replaced with sqrt(3), but that is a topic for another post. In your case, you are interested in the line-to-neutral resistance which is only the one-way resistance of the circuit. The 2 or sqrt(3) constant in the numerator disappears completely, and is now 1.
You also are given the fact that there are three conductors in parallel. This means three conductors are joined together at both ends, so that the current is shared among all three of them. Think about how three identical resistances combine in parallel, and apply it to your calculation. For other problems like this, think about how N identical resistances combine in parallel, to generalize this problem.
On top of all of that, there is another factor that comes in to play for AC circuits, called the skin effect, which is why it is relevant that it is AC, and why it is relevant that it is in a metal raceway. It turns out that since your choices are so far apart, that you can ignore this, and approximate through the formula you presented, modified to account for the factors discussed above. You may get an answer that is slightly different than the given options, but it will be close enough to conclude which of your options is correct.
There are resistance per unit length values in Chapter 9 / Table 9, that account for the skin effect, and you will notice that effective AC resistance is no longer inversely proportional to KCMIL area as it is for simple DC resistance. It is only a first order effect that resistance as inversely proportional to KCMIL area. The skin effect comes in to play, the larger the wire is, and it starts to matter for sizes 1/0 and greater. Metal raceways and the fact that it is AC, also increase this effect. It also matters for whether the metal raceway is aluminum or ferrous, as the ferromagnetism of steel raceways also impacts this. They neglected to specify what metal the raceway is, but I'd recommend assuming the metal is steel, as this is the most common in practice, and most likely what the author has in mind.
