The // operator in XPath is the abbreviated syntax for the descendant-or-self axis. When used after a node (e.g., //div//p), it selects all descendant nodes of the specified type (p) that exist at any level below the current context node (div). This allows for a comprehensive search through the entire sub-tree of an element without needing to specify the exact hierarchical path, matching the requirement to select all descendant nodes at any depth.

A. : This is a wildcard that selects all immediate children of the current node, not descendants at any depth.

B. |: This is a union operator used to combine the results of two separate path expressions into a single node-set.

C. @: This is the attribute axis specifier, used to select attributes of an element, not descendant nodes.

D. &: This is not a valid XPath operator for node selection; it is used for character entity references in XML/HTML.

1. W3C Recommendation, "XML Path Language (XPath) Version 1.0", Section 2.5 Abbreviated Syntax: " // is short for /descendant-or-self::node()/. For example, //para is short for /descendant-or-self::node()/child::para and so will select any para element in the document..." This source defines the operator's function as selecting descendants.

2. Micro Focus UFT One Help Center, "XPath and CSS expression syntax": The official documentation for UFT One (the tool associated with the exam) details the use of XPath for object identification. It specifies that // is used to "Select nodes in the document from the current node that match the selection no matter where they are." This directly supports its use for selecting descendants at any depth.

3. Stanford University, CS145 Lecture Notes, "XPath": Course materials on XML technologies frequently cover XPath syntax. In these notes, // is consistently defined as the operator for selecting descendants. For example, the expression //A is explained as selecting all elements anywhere in the document, which is a direct application of the descendant-or-self axis.