Let's start by figuring out how long the room is. We do this by looking at how long the ruler is. It appears to be 2 and 1/4th inches.
We divide 2 1/4th by 1/2 to get 4 1/2, so we multiply 4 feet by 4 1/2 to know that his room is 18 feet long.
Now, we do this except backwards to figure out how long the scale would be. Since 4 feet fits into 18 4 1/2 times, we multiply 2/3 by 4 1/2 to know how long the drawing would be. This equals 3, so the new drawing would be 3 inches.
Now we subtract 2 1/4 from 3 to get our answer, 3/4 of an inch, or C.