Based on this excerpt, Disney and Iwerks had a tough time getting their new business off the ground mostly due to

The correct answer is C the Leatherworkers Union strikes.

Explanation: In the context of their new business challenges, union strikes often pose significant obstacles to operations and financing. This suggests that external factors beyond their control, particularly labor disputes, were a primary reason for their struggles, rather than their personal qualities or the popularity of newsletters. If you need more detailed assistance, feel free to check the extended services page!