The correct answer is $\boxed{\$100}$.
Weighted average cost is calculated by dividing the total cost incurred by the total equivalent units completed. In this case, the total cost incurred is $350,000 and the total equivalent units completed are 3500. Therefore, the weighted average cost is $350,000 / 3500 = $100.
Option A is incorrect because it is the cost per unit of the beginning work in process inventory. Option B is incorrect because it is the cost per unit of the ending work in process inventory. Option C is incorrect because it is the cost per unit of the units started and completed during the period. Option D is incorrect because it is the cost per unit of the units transferred out of the work in process inventory.
I hope this helps!