Which option is checked by default to align the media file's source timecode?

The option that is checked by default to align the media file's source timecode is "Align Timecode."

When working with media files in Premiere Pro, aligning timecodes is crucial for ensuring that the timing of audio and video aligns perfectly, especially when editing multiple clips or layers. This alignment ensures consistency and synchronization across different pieces of media, which is essential for a smooth editing process. By having "Align Timecode" checked by default, the software facilitates a more seamless workflow, helping editors maintain the integrity of their media's timing without needing to adjust each clip manually.

Other options, while they may serve important functions, do not relate directly to the alignment of media timing. For instance, "Relink Others Automatically" is more about reconnecting offline media files while "Match Frame Rate" pertains to ensuring that the frame rates of different clips are compatible. "Replace Media" involves substituting one media file for another and does not focus on timecode synchronization. Therefore, the alignment of timecode becomes a primary function for editors working in a precise and organized manner, highlighting the importance of having this option enabled by default.