The MOST common method of procurement in the security field is a/an:

A Request for Proposal (RFP) is the most commonly used procurement method in the security industry, especially for complex systems where the buyer seeks not just price but also technical solutions, methodology, qualifications, and performance commitments.

A (RFI) is used for market research.

B (Invitation to bid) is used when scope is clearly defined and price is the primary factor.

D (RFQ) is for simpler purchases, focused on price.