This argument is saying that some union members say that the best course of action is an immediate strike. The author of the argument disagrees and cites the high costs as its reasoning.
The author of the argument is not considering whether the high cost is worth it or not. Without doubt, if the strike will have no positive impact then it is unsound to incur a loss. However, the author did not consider the possibility that the gains resulting from the strike will make the high cost worth it. This is answer choice C.
Answer choice B isn't the issue at play because knowing exactly what constitutes a major financial loss isn't necessary. The idea is that the union member thinks the loss is too high, whether it is $100,000 or $1,000,000 doesn't change this. It is all relative compared to the value they think a strike will bring.
I hope this helps. Please let me know if you have any other questions!