Which form is compared with اِسْتَعْصَمَ?

Ten Semantic Functions of Form X (Istifʿāl): Seeking, Suitability, Finding, Judging, Transformation, Adoption, Abbreviation, Response, Equivalence and Independent Use

• أَجَابَ — Form IV
• وَقَرَّ — Form I
• اِعْتَصَمَ — Form VIII
• تَكَبَّرَ — Form V