Which of the following is true when representing a $1$:$1$ binary relationship using the relational model?

Question $15$ options:

The key of the entity with the highest minimum cardinality must be placed in the other entity as a foreign key.

The key of each entity must be placed in the other as a foreign key.

The key of either entity is placed in the other as a foreign key.

The key of the entity with the most attributes must be placed in the other entity as a foreign key.