The first Tirthankara in Jainism is believed to be......

The correct option is Rishabhnath.

According to Jain tradition, a Tirthankara is a spiritual teacher and savior who has conquered the cycle of rebirth and defined a path for others to follow. Jainism recognizes a lineage of 24 Tirthankaras in the present half-cycle of time.

Rishabhnath (also known as Rishabhadeva or Adinatha, which translates to the "First Lord") is revered as the first Tirthankara of Jainism. He is traditionally believed to have lived millions of years ago and is credited with laying the foundations of human civilization, teaching humanity basic skills such as agriculture, writing, and arts, before dedicating his life to asceticism and achieving liberation.

To understand the context of the other options:
1. Sambhunath (Sambhavanatha) was the third Tirthankara.
2. Arishtanemi (Neminatha) was the twenty-second Tirthankara.
3. Mahavira (Vardhamana) was the twenty-fourth and final Tirthankara of the current era.