One more B. See below why I think A is wrong.
Although most of the hundred million bicycles in the United States are probably still relegated to garages and basements, the rising price of motor fuels has led to an increasing number being used for both transportation and recreation.
A. an increasing number being (Has led to what? To an increase, not to number)
B. an increase in the number (good)
C. increasing numbers that are (‘led to … numbers’ is wrong)
D. and increased number being (‘led to and increased number’ has no sense)
E. an increase in the numbers being (‘the numbers’ is wrong, also ‘being’ is redundant)