Comprehensive and Detailed Explanation:

On Windows PowerShell, gci is an alias for Get-ChildItem. To search recursively through all files and return matches for the string "ProjectX", the combination gci -Path . -Recurse | Select-String -Pattern "ProjectX" is efficient and returns file paths and matching lines. This handles large repositories and searches file contents rather than just file names.

Why D over C/B/A:

C (Get-ChildItem * | Select-String "ProjectX"): Works but lacks -Recurse so it may not descend into subdirectories unless Get-ChildItem is invoked with -Recurse.

B (dir /R | findstr): dir /R lists alternate data streams; it does not reliably search file contents and is less robust for large repos.

A (gc * | select "ProjectX"): gc (Get-Content) on * could attempt to load huge files into memory and select "ProjectX" is not a correct PowerShell pattern for searching content.

PT0-003 mapping: Domain 4 — using scripting/PowerShell to efficiently locate sensitive strings in large code/data sets.