Which form is أَجَابَ in the pair اِسْتَجَابَ وَأَجَابَ?

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

• Form IV (ifʿāl)
• Form VIII
• Form V
• Form VII