Back in November 2015, Microsoft announced some changes to their OneDrive plans. The most important of them was storage to be reduced from 15 GB to 5 GB, eliminating the unlimited space and change in some shopping plans Cited.
These changes caused storm among users. Due to this, in December Microsoft provided opportunity to users to keep their OneDrive storage up to 15GB by doing some simple steps.
That offer was made available a for limited time, and it rapidly spreading among users. Microsoft detailed the time limit for offer:

It is possible to receive gifts until January 31 of this year and Microsoft account is also required.

You can keep your 15GB OneDrive storage by hitting following link. Also be aware that this offer available till end of the January. After that you can’t claim your free storage. So hurry up!
Microsoft special offer to keep your OneDrive storage