Answer:
a.infrared radiation
Step-by-step explanation:
Forensic Document examination is an area of forensic science, its goal is to determine a document's authenticity with the use of technology to find as many visible or invisible details as possible, who wrote it, when and where can be determined.
In the case of no visible details, when the document has been altered, when it has obliterations or erasures infrared light can be used to reveal, for example, letters or words that have been written with different ink, altered or removed.
I hope you find this information useful and interesting! Good luck!