Which of the following is a common by-product of the animal industry?

Meat is a primary product of the animal industry, which encompasses the raising and processing of animals for consumption. The animal industry includes livestock such as cattle, pigs, sheep, and poultry, all of which are bred and raised specifically for their meat. This production not only focuses on yielding meat as the main product but also influences various practices and technologies within agricultural biotechnology to enhance efficiency, improve animal health, and ensure sustainable practices.

While agricultural fertilizer can sometimes be derived from animal by-products like manure, it is not a primary product of the animal industry itself. Similarly, plastic and textiles, while they may have some indirect connections to the agricultural sector, are not by-products of the animal industry as they stem from different processes and industries altogether. Thus, meat stands out as the most relevant and direct output of animal farming practices making it the correct answer.