Questions: Old St. Peter's Basilica has a transept to create a cross-shaped floor plan, separate the clergy from the faithful, allow more people near Peter's tomb, better illuminate the church interior.

The answer is: create a cross-shaped floor plan.

Explanation for each option:

1. Create a cross-shaped floor plan: This is the correct answer. The transept in Old St. Peter's Basilica, as in many other basilicas, was designed to create a cross-shaped (cruciform) floor plan, which is symbolic of the Christian faith.

2. Separate the clergy from the faithful: While the transept does create a division within the church, its primary purpose is not to separate the clergy from the faithful. This separation is more typically achieved by the chancel or the choir area.

3. Allow more people near Peter's tomb: Although the transept might incidentally allow more people to gather in the church, its primary purpose is not to facilitate access to Peter's tomb. The tomb of St. Peter is located beneath the high altar, and the design of the basilica as a whole accommodates this.

4. Better illuminate the church interior: The transept does not primarily serve the purpose of illumination. The lighting of the church interior is more directly influenced by the placement of windows, clerestory, and other architectural features designed for that purpose.

In summary, the transept's main function in Old St. Peter's Basilica is to create a cross-shaped floor plan, which is a significant architectural and symbolic feature in Christian church design.