Hadith of Jibril: Comprehension and application

What is the function of «إِنْ» in «إِنِ اسْتَطَعْتَ»?

The Hadith of Jibril: Islam, Iman, Ihsan, and the Signs of the Hour (Account 1)

• Vocative particle
• Conditional particle
• Conjunction
• Past verb