If the U. S dollar is strengthening, then:

a. it has been unpegged from other currencies.
b. the U. S has adopted a hard peg policy.
c. the U. S has purchasing power parity.
d. it has appreciated in terms of other currencies.

d.it has appreciated in terms of other currencies.

Explanation:

Currency appreciation occurs when the value of a currency increases. It leads to increase in demand of the currency against another currency as currencies are traded in pairs or against another currency.Appreciation of currency is caused by a number of factors such as investments, trade, economic policies, businesses, trad balance, etc.If the U.S dollar is strengthening then the currency of U.S has appreciated against other currencies.