Step 1: Understanding the projection rate.
If the film is projected at 90 feet per minute, we can calculate the number of frames per foot by dividing the number of frames in one minute by the number of feet covered in one minute.
Step 2: Calculating the frames per foot.
Given that the film is projected at 90 feet per minute, we divide this by the number of frames in one minute. Typically, there are 24 frames per second in film projection, so:
\[
\text{Frames per foot} = \frac{90 \, \text{frames/minute}}{1 \, \text{minute per foot}} = 24 \, \text{frames}.
\]
\[
\boxed{(B) \, 24 \, \text{frames}}
\]