Explanation:
The author's complaint about 'the boots that creaked' left Mr. Gessler shocked.

Reasons:

1. Mr. Gessler was a skilled shoemaker who took immense pride in his work. He was passionate about creating high-quality shoes that were comfortable and durable. When the author complained about the creaking boots, it came as a shock to Mr. Gessler because he had put his heart and soul into making them.

2. Mr. Gessler's main goal was to satisfy his customers and create shoes that met their expectations. He considered his craft as an art form and believed that a perfectly crafted pair of boots should not make any noise. Therefore, the author's complaint about the creaking boots went against Mr. Gessler's belief and left him surprised.

3. The author's complaint not only shocked Mr. Gessler but also hurt his feelings. He took the complaint personally as it questioned his skills as a shoemaker. Mr. Gessler believed that his shoes were of superior quality, and the complaint made him question his own abilities.

4. Additionally, the complaint about the creaking boots affected Mr. Gessler's reputation as a shoemaker. Word of mouth was crucial for his business, and a dissatisfied customer could potentially deter others from buying his shoes. This could have long-term consequences for his livelihood and financial stability.

5. Overall, the author's complaint about 'the boots that creaked' left Mr. Gessler shocked and disheartened. It not only challenged his craftsmanship but also affected his reputation as a skilled shoemaker.

Therefore, the correct answer is option 'D' - shocked.