When you develop test cases for a software system, which of the following best describes well-known methodologies or techniques that can be used to design effective test cases?

Introduction / Context:
Test case design is more effective when it uses systematic methodologies rather than ad hoc guessing. Over time, the testing discipline has developed a set of common techniques that help testers choose representative and high value test cases. These techniques aim to maximize coverage and defect detection within practical time and resource limits.

Given Data / Assumptions:

The system under test has both input domains and complex behaviors defined by requirements or design.
Time and resources are limited, so exhaustive testing is impossible.
Testers need structured methods to derive test cases from requirements and models.

Concept / Approach:
Equivalence partitioning divides input domains into classes where the system is expected to behave similarly, so one representative from each class can be tested. Boundary value analysis focuses on values at the edges of these partitions, where defects are more likely. Decision table testing models combinations of conditions and actions. State transition testing models systems as states and events, ensuring that all important transitions are tested. Use-case based testing derives test cases from user scenarios described in requirements.

Step-by-Step Solution:
Recognize that option a lists several standard techniques proposed in testing literature. Check that these techniques are all about systematically deriving test cases from requirements, designs or models. Note that option b describes random testing, which might find some defects but is not considered a methodology for systematic test design. Option c suggests exhaustive testing of all inputs, which is usually infeasible and not a methodology used in practice. Option d describes reuse of test cases without analysis, which can miss project specific risks and requirements.

Verification / Alternative check:
If you consult testing syllabi such as ISTQB or textbooks on software testing, you will find equivalence partitioning, boundary value analysis, decision tables, state transition testing and use-case based testing presented as core design techniques. This matches the description in option a and confirms it as the correct answer.

Why Other Options Are Wrong:
Option b lacks structure and does not ensure coverage or traceability to requirements.
Option c is not feasible for non trivial systems; the input space is usually too large.
Option d risks using outdated or irrelevant tests that do not align with the current system's behavior.

Common Pitfalls:
A common pitfall is to rely only on one technique, such as boundary value analysis, and ignore others that might reveal different types of defects. Another issue is to treat the techniques mechanically without thinking about risk and business impact. The best practice is to combine several design methodologies and prioritize tests based on risk analysis.

Final Answer:
Effective test design uses structured techniques such as equivalence partitioning, boundary value analysis, decision tables, state transition testing and use-case based test design derived from requirements.