Final answer:
The optimal development time for manual film processing at 68°F is 4 minutes.
Step-by-step explanation:
The optimal development time for manual film processing depends on the temperature. The question states that the temperature is 68°F. For this temperature, Option 3: 4 minutes is the correct answer.
When developing film manually, the time required for development varies depending on the temperature of the developer solution. Lower temperatures require longer development times, while higher temperatures require shorter development times. It is important to follow the instructions provided by the film manufacturer or consult a development chart to determine the correct timing.
Other factors that can affect the development time include the type of film being used and the desired level of contrast for the final image.