Which software development methodology integrates security testing throughout the development lifecycle rather than only at the end?

Correct answer: DevSecOps

Option D is correct because DevSecOps is specifically designed to embed security practices, including automated security testing, code analysis, and compliance checks, continuously throughout the software development lifecycle rather than treating security as a final gate. Option A is incorrect because RAD focuses on rapid prototyping and quick iteration for user feedback and does not specifically mandate continuous security integration. Option B is incorrect because Waterfall is a sequential model where testing, including security testing, is typically deferred to late phases, making early vulnerability detection difficult. Option C is incorrect because the Spiral Model incorporates risk analysis but does not specifically mandate that security testing be integrated throughout all development phases the way DevSecOps does.