Going through a very basic children's Thai grammar book, I found the following exercise question (presumably) asking the reader to pick out one of three words containing a particular vowel:

คำในข้อใดมีสระ .. ...

Then there are three very simple words, one with the correct answer.

Its the structure of the question that seems a bit difficult. Translating directly you would get:

"word in case/instance what has vowel....." or in more normal form "Which word contains the vowel....".

So does this structure apply to every question of this sort. For example, would the following follow the same form:

"Which tin contains beans..". (the red tin, the green tin, the blue tin etc)

กระป๋องในข้อใดมีถั่ว.....

Thanks.

The structure is used to emphasise that the answer must be selected from the list of options.

คำในข้อใดมีสระ

In which option/answer does the word have a vowel?

Compare with:

คนในคันใดมีแว่น

In which car does the person have glasses?

กระป๋องในข้อใดมีถั่ว.....

doesn't make sense: In which answer does the can have beans?

I can't answer your overall question but one thing you're missing is

ข้อ - item ; provision ; clause (ข้อ is the classifier for a point, item or clause (e.g. in a contract).)

Which can has beans? =

ในกระป๋องใดมีถั่ว or

ในกระป๋องกระป๋องใดมีถั่ว

The quantifier "ใด" goes with the classifier, not the noun. However, if the classifier for the noun is the same as the noun itself, the noun may be omitted.