The basis on which the assumptions of a study are formulated is

The assumptions of a study rest on all three bases, so All of the above holds.

The cultural background of the setting shapes what can be taken for granted.

Knowledge available in universities and past studies grounds assumptions in prior work.

The specific characteristics of the group under study inform context-bound assumptions.

Each single basis alone gives only a partial foundation for assumptions.

Assumptions are accepted as true without proof but must remain reasonable and justified.