The source used to obtain the image in Optical Coherence Tomography is:

In Optical Coherence Tomography (OCT), the source used to obtain the images is typically based on light, specifically through the use of coherent light. While the term "high resolution imaging" generally describes one of the key benefits of OCT technology, it does not refer to the source of the imaging itself. The imaging in OCT is accomplished by using light waves, often from sources like near-infrared lasers or broadband light sources, which allow for detailed cross-sectional imaging of biological tissues, especially in ophthalmology.

The process relies on the interference patterns created by light reflecting off of different layers within the tissue, enabling high-resolution depth information to be acquired. The correct understanding here is that OCT utilizes light for imaging but the term should directly relate to the specific type of light used, which is neither broadly described as "high resolution imaging" nor does it refer to ultrasound energy or specific colors of light as indicated in the other options.

In conclusion, while "high resolution imaging" does reflect an important aspect of what makes OCT valuable, it is crucial to recognize that the technology fundamentally operates by using coherent light sources, which allows for the generation of the detailed images characteristic of OCT.