mistake with destroyer

Actually it's not wrong.

The generic form of an Acetal used to be considered as you mentioned or as displayed below:


But many have interpreted today that an Acetal can be considered a Carbon bonded with 2-OR groups and 2-R groups. Which in fact is a Ketal as shown below:


But todays terminology in Organic Chemistry all ketals are classified under a subset of Acetals. So in fact the answer is correct. It's a trick question but nothing wrong with it.