What must be used to close abandoned inlet openings in chimneys and vents?

The correct answer is found in the necessity for using an "approved method" for closing abandoned inlet openings in chimneys and vents. This approach ensures that safety regulations and building codes are strictly adhered to, minimizing risks associated with improper sealing that could lead to issues like drafts, backdrafting of combustion gases, or structural failure of the chimney or vent system.

By using an approved method, the process is typically vetted by relevant local codes or standards, ensuring that the materials and techniques employed are reliable and safe. This is particularly important as chimneys and vents are integral to the ventilation of appliances and the safe expulsion of combustion gases.

Other options, while they might seem feasible, may not meet the required safety and regulatory standards. For example, a temporary cover might not provide a long-lasting seal, and a concrete block could present issues with movement or cracking, depending on conditions. A sealed cap is a specific type of solution but may not be universally accepted or appropriate in all cases, whereas leaning on what is classified as an "approved method" ensures compliance and safety.