There is a plain B-tree index on users (email). Which query cannot use it?

The correct answer is SELECT * FROM users WHERE lower(email) = 'a@x.com'. The index stores email values, not lower(email) values. When you apply a function to the column, the index can no longer match it. Fix it with an expression index: CREATE INDEX ON users (lower(email)). Then the query can use it.

The other queries use the bare email column. A B-tree handles IN, ranges and ordering on the bare column.