While the bustling city streets were filled with the cacophony of honking horns and chatter of pedestrians, the serene countryside, where time seemed to move at a slower pace, provide a welcome retreat for their seeking solace amidst the chaos of urban living.

There are a few errors in the underlined part of the sentence. First among them is tense inconsistency: he first clause uses past tense ("were filled"), but "provide" is present tense. It should maintain the past tense for parallelism. The second is the error in using the pronoun "their" to refer to people in general. The correct should be "those". 

So the correct answer should be " provided a welcome retreat for those seeking solace", which is option D. This is the correct answer.

Option A seems close,  but "a welcome retreat" is more natural than "a welcoming retreat" in this context. "Welcome" functions as an adjective meaning "pleasing and wanted," while "welcoming" would describe the retreat's behaviour.