c. Overproduction was causing prices to drop.
Explanation:
The Texas oil producers were ordered to limit production during the Great Depression because "Overproduction was causing prices to drop."
This is because, during the great depression period around the 1930s, there was less demand for oil and at the same, there was overproduction of oil from Texas producers due to discoveries of oil in abundance.
Hence, the need to limit the production of oil to stabilize the price and reduce the huge production loss.