The correct answer is **C. were able to use**.

Here's the reasoning:

- **A. could use** - This implies a present ability or a general possibility but doesn't fit the context of a past event.
- **B. could have used** - This suggests a potential action in the past that did not happen, which doesn't align with the sentence indicating that they did indeed use the gas cooker.
- **C. were able to use** - This indicates that they had the ability to use the gas cooker in the past, which fits perfectly with the context of the sentence as it describes a specific action they took that evening.
- **D. may have used** - This suggests uncertainty or a possibility about a past action, which does not match the context since the speaker clearly states they were let into the kitchen and had the opportunity to use the cooker.

Thus, **C. were able to use** is the most appropriate choice that conveys a completed action in the past.