1. This is only one instance which/where it was not safe to eat genetically engineered foods.

2. Sometimes this caregiving occurs in three-generation households which/where the oldest generation contributes to the household economy.

3. Air transportation is an essential ingredient of tourism development, especially at an international scale which/where destination choices may be constrained by time.

All the answers provided by my book is 'where'. I don't know how to understand and teach this. (My book says the reason the answers are where is the parts after where are complete clauses)
moon7296(My book says the reason the answers are where is the parts after where are complete clauses)
Whoever wrote your book knows precious little about English syntax. Which can begin a "complete clause" but it is normally the subject or object in that clause while where cannot assume that role.
moon72961. This is only one instance which/where it was not safe to eat genetically engineered foods.
It is the grammatical subject, so having which before it is just plain impossible. A relative clause can't have two subjects. This is an example of a grammatically correct which: This is only one instance which couldn't be explained.
moon72962. Sometimes this caregiving occurs in three-generation households which/where the oldest generation contributes to the household economy.
If we use which, we get: Sometimes this caregiving occurs in three-generation households which the oldest generation contributes to the household economy.

The sentence is grammatical but doesn't make much sense. It says that the oldest generation contributes three-generation households to the household economy. Which is the object of contributes and the antecedent of which is three-generation households.
moon72963. Air transportation is an essential ingredient of tourism development, especially at an international scale which/where destination choices may be constrained by time.
Destination choices may be constrained by time would be in itself a complete sentence and a typical English passive structure. Destination choices is the grammatical subject even though it is actually the object of constraining. We cannot add another subject (which) before it.

I get the impression from your post that you teach English. I understand that teaching a language is very difficult if one doesn't know the language very well oneself. All you need to do is continue your English studies!

I wish you a lot of success.

CB
Hi,

I'm preparing an exam to be a public school English teacher. I found I need some strategies to teach English other than things I had done to learn English only for myself.

I understand quite a lot about where/which but need to know more to teach them clearly.

Thank you for your long answer.