Take context from the input text and decide on the accurate relationship between 'Windows Phone' and 'Microsoft' from the options provided.
Options: publisher, product/material produced, headquarters location, developer

Here are six important things you should know about Microsoft Windows 10... Microsoft announced that Windows 10 will be free for customers running Windows 7, Windows 8.1, and Windows Phone 8.1 if they upgrade in the first year after launch.
developer