$23.$

Your team implemented large custom software about six months ago and the system has been live in Production since then. You are in$-$charge of ensuring that the system is stable and all enhancements and$/$or changes do not adversely affect the Production system. What is the MOST IMPORTANT step you can take to ensure that you keep your system from failing due to any future change$/$enhancements$?$

Only allow changes that were certified as having followed the best practices of software engineering principles

Only allow changes from the most well$-$known software companies with the best reputation

Perform very thorough unit$-$tests, user$-$acceptance$-$tests and system integration tests in a test environment that mimics the production system and only allow the changes that have certifiably passed to be deployed

Only allow changes that have been coded$/$developed in the same technology as that the base production application