Question

Which statement describes why radiocarbon dating has an upward limit of 50 000 years

Answer 1

Because  The half-life of carbon-14 is around 5,700 years. So, by measuring the half-live of the sample, we could predict how old the sample actually is.
After 50,000 years there is  probably less than 1% of the original C-14 left which make it much more accurate to measure, so, the upward limit is put on 50,000 years.