Is this the answer to "What is shopping?" ?
I would have expected the question to be "What is your most hated thing?" and so for "siopa" to come at the beginning in the identification sentence: Siopa ydy fy nghas beth.
How common is the term "cas beth" used in the South? I don't think i've ever heard it used up North. Far more natural would be "Y peth dwi'n casáu mwyaf yw..."