Answer: (iii) Serve as a contract between client and developer

The Software Requirements Specification (SRS) is a formal document that completely describes the intended purpose, behaviour, and constraints of the software. Its main purpose is to act as a contract/agreement between the customer (client) and the developer regarding exactly what the software will do — serving as the unambiguous reference for design, development, testing, acceptance, and payment.

Why the others are wrong:

• (i) Describe coding standards — coding standards are separate process documents; the SRS is about what to build, not how to code it.
• (ii) Provide a user manual — a user manual describes the finished product to end users; the SRS is written before development.
• (iv) Define the software test cases — test cases are derived from the SRS (testers use it as a basis), but defining them is a later testing activity.

An SRS therefore captures the complete requirements — functional and non-functional — and underpins the client–developer contract.