How to use contact builder in Marketing cloud . I have created Synchronized Data Extensions . But I am not be able to use in journey builder Because there is no option of it .
Synchronized data extensions aren't available as primary sendable data extensions (see here: https://help.salesforce.com/articleView?id=mc_cab_implement_synchronized_data_sources.htm&type=5). Now that you have your synchronized data extension, you will need create a filtered data extension (that is sendable) or use a query activity to populate a sendable data extension for your Journey.