Answer: A the mythic ideas of what the city was built on
Explanation;
By stating that the Greek and Roman heroes laid the metaphoric foundations for what became major cities, the text seems to infer that the heroes laid the mythical ideas of what the city was built on.
Metaphors are used to describe things in a way that are not literally applicable so when it is said that the foundations laid were metaphoric, it meant that the foundations did not mean actual physical foundations but something else which in this case meant the mythic ideas of what the city was built on.