Reference required to assembly 'Microsoft.SharePoint.Client.Runtime,version=16.0.0.0

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1

reference required to assembly ‘Microsoft.SharePoint.Client.Runtime,version=16.0.0.0 culture=neutral,public token=785d84c84’ containing the base class ‘Microsoft.SharePoint.Client.ClientRuntimeContext’.add one to your project

in SharePoint Custom Activity

How to resolve this ?can any one help this?