What type of inspections are conducted under progressive maintenance?

The correct answer is that progressive maintenance involves conducting four-phase inspections every 60 hours. This method of maintenance allows for a systematic approach to aircraft inspections, where the total inspection workload is divided into smaller, manageable segments.

In progressive maintenance, each phase typically covers specific components and systems of the aircraft, ensuring that all necessary checks and maintenance are performed on a more frequent basis rather than all at once during an annual inspection. This not only helps in maintaining the aircraft's airworthiness but also allows for continuous oversight and timely detection of any potential issues.

While the other choices mention specific types of inspections, they do not align with the comprehensive, organized structure required for progressive maintenance, which is designed to improve operational efficiency and safety through regular, detailed inspections at defined intervals. Each of the other options focuses on limited aspects of maintenance that do not encapsulate the complete nature of progressive inspection practices.