Final answer:
The taxpayer cannot claim the grandchild as a dependent on their tax return.
Step-by-step explanation:
In this case, a taxpayer's grandchild is age 18, married, a resident of Canada, and files a separate tax return. Based on this information, it can be determined that the grandchild is not a dependent of the taxpayer.
When it comes to claiming dependents on a tax return, there are certain criteria that must be met, such as relationship, age, residency, and support. Since the grandchild is married and files a separate tax return, they would not meet the criteria to be claimed as a dependent by the taxpayer.
Therefore, the taxpayer cannot claim the grandchild as a dependent and neither QC (Qualifying Child) nor QR (Qualifying Relative) would apply in this situation.