What is a diplomat who resides in a foreign country where he represents his home country called?

A diplomat is someone selected by his nation that represents and protects his nations' interest. Between the diplomats there are 4 different types; ambassadors, envoys, ministers and charge' affairs. Considerring that the question specifies that the diplomat need to reside in a foreign country the correct answer would be; ambassador.

An ambassador is the diplomat selected by his country who will represent his nation in a foreign country. Therefore we can conclude that "Ambassador" is the correct answer for your question.