How to enforce the shape of a JSON/JSONB in Postgres?

There 'right' way yo do this is either to normalize your data or to enforce schema at the application level.

Postgres JSON is delibrately schemaless. The idea is that if your data is structured it should probably be in normal columns. However, JSON is a totally valid option in the situation where the data is structured but the schema is dynamic. In those situations it's best to enforce schema constraints at the application layer.

In general, the topic is extensive and complex. For those interested, I recommend the JSON Schema website.

For our purposes, we can use a very simple method of validation, limited to two issues, commented in the function body:

create or replace function validate_answer(answer jsonb, pattern jsonb)
returns bool language plpgsql as $$
declare
    rec record;
begin
-- does the answer contain exactly the same keys as the pattern?
    if not (
        select array_agg(keys_ans) = array_agg(keys_pat)
        from (
            select 
                jsonb_object_keys(answer) as keys_ans, 
                jsonb_object_keys(pattern) as keys_pat
            ) s
        ) then return false;
    end if;

-- are the value types of all keys the same in the answer and pattern?
    for rec in
        select *
        from jsonb_each(pattern)
    loop
        if jsonb_typeof(answer->rec.key) <> jsonb_typeof(rec.value)
        then return false;
        end if;
    end loop;
    return true;
end $$;

Test the function in Db<>Fiddle.

As you can see, the pattern is just an example of a well-formatted answer. Define a pattern for each question, create a trigger for the answers table and use the above function inside the trigger function to verify new or modified answers.