Final answer:
The default search text in a Knowledge Search from an Incident record is typically taken from the Summary field. This field provides a coherent description of the incident, making it effective for matching with relevant knowledge articles. Therefore, the correct option is b. Summary field.
Step-by-step explanation:
When performing a Knowledge Search in an Incident Management system, the default search text is usually taken from the Summary field of an incident record. This field generally contains a concise but comprehensive description of the issue, which makes it suitable for aligning with knowledge base articles that could provide solutions. The Summary field acts as a starting point for the search, which can then be refined, if necessary, by adding more specific search terms.