The statement that is incorrect about the sect of the Ajivikas is option 'A' which states that it attracted only men of humble origin. The Ajivikas sect was actually open to people from all backgrounds and did not discriminate based on social status or gender.

Explanation:
1. The Ajivikas sect:
The Ajivikas sect was an ancient Indian religious sect that originated around the same time as Buddhism and Jainism. It was founded by Makkhali Gosala, who was a contemporary of Gautama Buddha and Mahavira, the founder of Jainism.

2. Attraction of people from diverse backgrounds:
Contrary to option 'A', the Ajivikas sect did not attract only men of humble origin. The sect was open to people from all social backgrounds, including both men and women. It did not discriminate based on social status or gender, which was a unique aspect of the Ajivikas sect.

3. Criticism of the Brahman interpretation of Karma doctrine:
Option 'B' correctly states that the Ajivikas sect found fault with the brahmans' interpretation of the Karma doctrine. The Ajivikas had their own interpretation of the concept of Karma and criticized the brahmans for their understanding and application of it.

4. Criticism of the caste structure:
Option 'C' correctly states that the Ajivikas sect criticized the caste structure. The Ajivikas believed in the doctrine of fatalism, which means that they believed that everything in life was predestined and predetermined. Therefore, they criticized the caste system as they believed that one's social position was not determined by their actions or merits but was predetermined by destiny.

5. Founder of the Ajivikas sect:
Option 'D' correctly states that Gosala was the founder of the Ajivikas sect. Makkhali Gosala was a contemporary and a rival of Gautama Buddha and Mahavira. He founded the Ajivikas sect and was known for his teachings on the concepts of destiny, fate, and determinism.

In conclusion, the statement that is incorrect about the sect of the Ajivikas is option 'A' which states that it attracted only men of humble origin. The Ajivikas sect was open to people from all backgrounds and did not discriminate based on social status or gender.