To find the cost per chair, you would want to divide the total cost by the number of chairs. This means you need to use the expression:
\[ \frac{150 \text{ dollars}}{50 \text{ chairs}} \]
Thus, the correct expression from your options is:
\[ \text{StartFraction 150 dollars Over 50 chairs EndFraction} \]
This expression will yield the cost per chair when calculated.