The correct answer is: D. both (a) and (b)
Structured design is a software design approach that uses top-down decomposition and stepwise refinement to break down a complex system into smaller and smaller modules until each module can be easily understood and implemented. This approach helps to produce computer programs that are both easily maintained and easily understood.
Option A is correct because structured design helps to produce computer programs that are easily maintained. This is because structured design breaks down a complex system into smaller and smaller modules, which makes it easier to identify and fix bugs. Additionally, structured design uses top-down decomposition and stepwise refinement, which helps to ensure that the modules are well-defined and easy to understand.
Option B is also correct because structured design helps to produce computer programs that are easily understood. This is because structured design breaks down a complex system into smaller and smaller modules, which makes it easier to follow the logic of the program. Additionally, structured design uses top-down decomposition and stepwise refinement, which helps to ensure that the modules are well-defined and easy to understand.
Option C is incorrect because structured design does not necessarily produce computer programs that are easily understood by end users. This is because structured design is a software design approach, and it does not take into account the needs of end users. However, structured design can help to produce computer programs that are easier to maintain and understand by programmers.
Option E is incorrect because structured design is a valid software design approach. It has been used successfully to produce many complex computer programs.