What is needed as a result of different countries having different currencies?
a.
business plans
b.
peace treaties
c.
free trade agreements
d.
a system of exchange

D.a system of exchange

Explanation:

When different countries have different currencies you need to set up a system of exchange in order to see how much is the currency of a given country worth in other country, so people can buy or exchange their currency to be able to buy products in the other country, for example the US Dollar is a currency widely accepted through the world, many establishments would accept dollars even if their local currency is different, and in many cities around the world you can find exchange shops that would buy your US dollars for the local currency.

b - the respond is innate and varies over time.

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"a.) well–developed vascular system."