The correct option is In the Backlog view open the item context menu add a link choose Successor and enter the ID of taskY.

Using the Backlog view and the item context menu lets you create a Successor link that explicitly models that taskX must wait for taskY to finish. The Successor link type is a first class relationship in Azure Boards for sequencing work and the backlog and dependency visualizations respect that relationship.

Add a Parent link from the user story that contains taskX is incorrect because a Parent link creates a hierarchy to show decomposition of work and it does not express sequencing or a finish-to-start dependency between two tasks.

From the work item form open the Links tab add a link and use References with the ID of taskY is incorrect because a References link is a generic connection for context and notes and it does not establish a predecessor or successor relationship that the backlog will treat as a dependency.

From Queries open the context menu add a link choose Existing Item set the link type to Affected By and enter the ID of taskY is incorrect because the Affected By link denotes impact rather than an ordered dependency and it is not the standard predecessor or successor link used to express sequencing in the backlog.

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