The correct answer is 'Writing pseudocode'. Pseudocode is an informal high-level description of the operating principle of a computer program or an algorithm. It focuses on the flow of the program and algorithmic principles rather than specific syntax of a language, making it easier to transcribe into actual code later. 'Flow-chart concepts' might initially seem correct, but pseudocode is typically better for breaking down problems into smaller steps in text form. 'Developing a vector' and 'Designing a user interface' are important in later stages of program development or for specific tasks.