Option 1 states that Singapore is known as Port of Call because it deals in the processing and shipping of oil. However, the provided answer states that Singapore is known as Port of Call because it is on the main sea route where ships use to anchor for refueling, watering, and taking food items. These reasons are more closely related to the concept of a Port of Call.

Option 2 suggests that Singapore is a deep water port built away from the actual port. However, this option does not explain why Singapore is known as a Port of Call.

Option 3 is the correct answer. It explains that Singapore is known as Port of Call because it is located on the main sea route where ships anchor for essential activities such as refueling, watering, and obtaining food items. This concept aligns with the definition of a Port of Call.

Option 4 states that Singapore serves the parent ports by receiving ships that are unable to approach due to their large size. While this statement may be true, it does not directly explain why Singapore is known as a Port of Call.

In summary, option 3 is the correct answer because it best describes the reason why Singapore is known as Port of Call.