Reasonable request, usually. Some, though, verge on the bizarre. One brandname media outlet had a three-hour test. There was no compensation. And many were scheduled for that exercise. Therefore, the odds of investing one's time and having a payoff were not good.
In addition, you don't know if there really is an opportunity involved. Or, are they just fishing for ideas they can receive without paying?
That's exactly why the risk should be shared. Before you comply with the mandate for a custom-made sample, first ask for a kill fee if the content you provide is not accepted as evidence that you can do the job. Business is based on risk-sharing. If the organization is not in that mindset, no, you don't want to go any further with the application. It's likely you will not be treated well.