Final answer:
The animals run into harsh weather conditions when building the windmill.
Step-by-step explanation:
The problem the animals run into in building the windmill is C) Harsh weather conditions. In the book 'Animal Farm' by George Orwell, the animals on the farm decide to build a windmill to generate electricity. However, they face challenges due to the harsh weather conditions, such as strong winds and heavy rain, which make it difficult to construct the windmill. These weather conditions cause setbacks and delays in their construction efforts.