Explanation:

✅ B. BuildRowSetFromString
This AMPscript function splits a delimited string (such as a tab-delimited list) into individual elements and creates a rowset. You can then iterate through or search the rowset to see if a specific value exists. This is the most flexible and scalable method for checking membership in a delimited string, particularly for tab-separated values.

❌ A. Substring
Substring extracts a portion of a string based on a starting index and length. It is not suitable for searching within a delimited list, especially when trying to detect if a value exists across different positions.

❌ C. IndexOf
IndexOf returns the position of the first instance of a substring in a string. While it could technically find if a string exists, it's not robust for tab-delimited lists—e.g., it might return a false positive if a substring exists as part of a larger value.

❌ D. Length
Length returns the number of characters in a string. It does not perform any search or matching operations, so it’s not relevant to checking whether a specific string exists.