When a country is not at war or engaged in hostilities, it is said to be at ...

- rest

- armistice

- conflict

- peace

- definitely not conflict; as this would be a hostility!

-armistice means an interruption in fighting, but it does not mean an end of a war: so also not a good option

- I have never heard that a country could be "at rest" ...

but I have heard of countries at peace, and this is the correct option!