Answer: Slmgr /<sideloading product key>
Step-by-step explanation:
Side loading is referred as the procedure of the installation of the Window store application without the interference of the Window store .Side-loading is done by the method of side-loading activation key in a computer system .The activation is done by starting of the command prompt screen and then using the command "Slmgr /<sideloading product key> " for the activation of the key.
Therefore , the correct option is Slmgr /<sideloading product key>