Which statement about fastener standards is true?

Military standards are indeed used in the aerospace industry, particularly for components that must meet rigorous safety and performance requirements. Aerospace applications demand high reliability and consistency due to the critical nature of the environments in which these fasteners operate, such as extreme temperatures, pressures, and stresses. Military standards, such as those set by the Department of Defense, specify strict guidelines that must be adhered to, ensuring that fasteners used in aerospace meet the highest levels of quality and durability. This alignment helps ensure not only the safety of the aircraft but also compliance with necessary regulations and best practices in manufacturing.

In contrast, while industrial standards may be relevant in various fields, they are not as universally stringent or specifically tailored for aerospace applications. Fasteners do have standard metrics; they are designed and categorized based on specific measurements and specifications. Lastly, fastener standards are generally not optional in manufacturing, especially in industries like aerospace, where adherence to such standards is crucial for safety and performance. This context reinforces why the statement regarding military standards is the correct answer.