Utilize the input text as a context reference, choose the right relationship between 'Walter Energy' and 'coal' from the options.
Options: product/material produced, owner of, brand, stock exchange

The commodity rout is also putting pressure on stronger coal companies, which have weathered a difficult time for the sector in the last two years that forced smaller companies, such as Walter Energy Inc, into bankruptcy protection and more are expected to follow.
product/material produced