An offer that specifies a time for acceptance is:

An offer that specifies a time for acceptance is irrevocable during that specified period. This means that the offeror cannot withdraw the offer until that time has elapsed. The principle behind this is that when a time frame is established, the offeree is afforded an appropriate period to consider the offer without the concern that the offer might be rescinded prematurely.

In contract law, clarity and certainty are key elements. By establishing a stipulated timeframe, both parties are aware of the grounds for decision-making. If the offer is not accepted within the designated period, it will naturally expire, but until that moment arrives, the offer remains binding.

The other options do not accurately reflect this legal understanding. An offer that is revocable at any moment would undermine the time-bound nature established in the question. Likewise, specifying that an offer is void after a short period like 48 hours does not align with the concept of a defined time frame unless explicitly stated in the offer. Lastly, an automatic acceptance if no response is given contrasts directly with the notion that parties must actively agree within the specified time.