Sampling methods

Not sure what you mean – types of audit sampling are statistical and non-statistical and sample selection methods are interval, monetary unit, haphazard, etc (see Chapter 11 of notes). I don’t think you are going to be asked to specify a particular method of sample selection for a particular audit objective in AA.

Do you mean to ask from where should the sample be taken? (i.e. what is the population?)

Regardless of the class of transaction or account balance you are looking at – for the assertion of completeness you are asking “Is the recorded amount complete?”/”Has everything that should be included in this total been included in this total?” So to test the completeness of recorded inventory you would select (a sample of) items from PHYSICAL and trace/check/agree the quantity recorded (e.g. on the stock sheets).
For an account balance – the assertion of existence means just that – does this recorded asset/liability exist? So here you would select (a sample of) items from the RECORDED quantities to the actual physical quantities.

The “direction” of the test – “physical to book” or “book to physical” – depends on the assertion.