Final answer:
Memory usage is an example of a behavior that requires dynamic analysis to understand because it involves observing the program as it allocates and deallocates memory during its execution.
Step-by-step explanation:
An example of program behavior that can be learned only through dynamic analysis is c) Memory usage. Dynamic analysis involves running the program and observing its behavior during execution. Syntax errors, which relate to code grammar, can be identified through static analysis without running the program, hence, they are not specific to dynamic analysis. The speed of a program (Execution speed) often relies on dynamic analysis to measure real-time performance, but it is the memory usage that truly requires this form of analysis because memory allocation and deallocation occur as the program runs and cannot be fully predicted statically. Similarly, Algorithm complexity is more of a theoretical measure and can be analyzed statically through code inspection.