The most appropriate type of legal contract to enforce confidentiality during preliminary negotiations is a Non-Disclosure Agreement. It legally binds the parties to keep certain shared information confidential, thereby protecting the company's proprietary information, such as trade secrets or strategic plans. While a Letter of Intent can demonstrate commitment to a deal, it does not typically bind parties to confidentiality. An Employment Agreement is used between an employer and employee and is not suitable for negotiations with a potential partner. A Master Services Agreement sets forth terms of a working relationship, typically finalized after trust has been established and does not necessarily ensure confidentiality at the initial stages of discussion.