How is it possible to make a profit from buying and selling currency?

a. the value of currency always stays the same, but you can exchange money for currency that is more valuable to make a profit.
b the value of currency changes over time, so you can buy a country’s currency when its value is low and sell it when it is higher.
c the value of currency changes over time, so you can buy a country’s currency when its value is high and sell it when it is lower.
d the value of currency is always going up, so you can buy currency, hold onto it, and then sell it later to make a profit.