Designing a questionnaire is a sequence in which each step supplies what the next one needs, so the question is whether any listed step can be omitted.

Step (a) is required and comes first. Until the primary and secondary aims of the study are written down, there is no basis for deciding what to ask; every item in the questionnaire must serve one of those aims, and an instrument built without them collects data that answer no question.

Step (b) is required. Reviewing the current literature shows how others have measured the same constructs, supplies tested items and wordings that may be adopted or adapted, and prevents the researcher from repeating known mistakes or overlooking an important dimension.

Step (c) is required. The draft turns the aims and the borrowed insight into actual questions, with decisions on wording, sequence, response format and length.

Step (d) is required. No first draft is adequate. Revision follows expert comment and pre-testing on a small group, and it removes ambiguous, leading or redundant items and corrects a flawed sequence before the instrument goes into the field, where errors can no longer be repaired.

Since all four steps are necessary and sequential, the fourth code applies.

Hence all the steps (a), (b), (c) and (d) are required.