We should focus on only one thing in the

Solution

The phrase "at a time" is used to indicate that something is done or considered in a specific period or instance. It implies that we should concentrate on a single task or activity at any given moment. The use of "in the time" is incorrect in this context. It doesn't convey the intended meaning of focusing on one thing at a specific point in time.Therefore, "at a time" is the appropriate prepositional phrase to use in this sentence.