Final answer:
The appropriate answer is 'had to buy', as it indicates a past necessity or obligation to purchase despite the high exchange rates.
Step-by-step explanation:
The correct answer to the question is: Even though the exchange rate was high, we had to buy from them. The sentence indicates a past necessity which rules out option A (buy) as it is present tense.
The appropriate answer is 'had to buy', as it indicates a past necessity or obligation to purchase despite the high exchange rates.
Option B (must have bought) suggests a deduction or assumption which is not implied in the context. Option D (better buy) is giving advice which again does not fit the context. Option C (had to buy) correctly implies a necessity or obligation in the past, therefore, it fits the sentence structure and meaning appropriately.