Hello Francisco,
The "Benefit Measurement Method" and the "Constrained Optimization Method" are both project selection methods.
However: Project selection methods have been removed from the PMBOK Guide 4th Edition. Because of this you can safely assume that they are no longer on the PMP exam and you do not need to know these two methods in detail. As long as you understand that these two terms are project selection methods you will be fine (in fact... you are over-prepared).
But to answer your question:
Benefit Measurement is a comparative approach and examples would be scoring models, cost-benefit analysis, review board, economic models. Benefit measurement is the most common approach.
Constrained Optimization is a mathematical approach and examples would be linear programming, nonlinear programming, integer programming, dynamic programming, multi-objective programming. Constrained optimization makes use of math models and complex criteria and is often managed as a distinct project phase.
But remember: You should not need this knowledge any longer. The new PMBOK Guide discusses project selection on a high level (see chapter 2) and doesn't go much more into specifics other than saying that "every project should support your company's strategic plan".
