The correct answer to a methodology for handling large and complex projects is the Waterfall Model.
And the reason for this is, since in Agile and other methodologies the requirements change from time to time, and can change at any stage in the project. It is difficult to accommodate these changes when you have large and complex projects.
Therefore waterfall model is the apt choice for this issue. since in this, the requirements do not change once the requirement stage is over.
For more information, refer to this DevOps Tutorial
Also, learn how to incorporate the spiral model to increase efficiency and flexibility.