A change in investors' perspectives that makes maintaining a fixed exchange rate untenable is called:a speculative attack.a revaluation.the impossible trinity.a risk premium.

A change in investors' perspectives that makes maintaining a fixed exchange rate untenable is called a speculative attack.

Here's why:

1. A speculative attack in the context of international finance is a situation where traders start selling off a country's currency in anticipation of a future devaluation.

2. This usually happens when investors believe that a country's central bank does not have enough foreign exchange reserves to maintain the country's fixed exchange rate.

3. The selling pressure from traders can make it very difficult for a central bank to maintain the fixed exchange rate, leading to a devaluation of the currency.

4. Therefore, a change in investors' perspectives that makes maintaining a fixed exchange rate untenable is referred to as a speculative attack.

The other terms mentioned - revaluation, the impossible trinity, and risk premium - are related to exchange rates and international finance, but they do not specifically refer to a situation where a change in investors' perspectives makes maintaining a fixed exchange rate untenable.