ANS:E - all the above.

The minimum thickness of a flat slab depends on various factors including the span length, the presence of drops, and design considerations related to deflection, punching shear, and overall structural performance. Generally, the minimum thickness of a flat slab is specified to ensure adequate strength, stiffness, and serviceability of the structure. The options provided include:

1. 13 cm: This could be a specific minimum thickness requirement specified by a design code or standard for certain types of flat slabs. However, without additional context, it is difficult to determine the applicability of this specific thickness requirement.
2. L/32 for end panels without drops: This is a common guideline for determining the minimum thickness of end panels of flat slabs without drop panels. The thickness is typically expressed as a fraction of the span length (L), where L/32 is often used as a conservative estimate to ensure sufficient strength and stiffness.
3. L/36 for end panels without drops: Similar to the previous option, this guideline specifies a minimum thickness of end panels of flat slabs without drop panels, expressed as a fraction of the span length (L). L/36 is another commonly used value for determining the minimum thickness of end panels, providing a slightly thinner slab compared to L/32 while still ensuring adequate performance.
4. L/36 for interior panels without drop: This option refers to the minimum thickness requirement for interior panels of flat slabs without drop panels, expressed as a fraction of the span length (L). Interior panels typically have shorter spans compared to end panels, allowing for a slightly thinner slab thickness to be used while still meeting strength and stiffness requirements.

Therefore, the correct answer is all of the above. The minimum thickness of a flat slab can vary based on the specific design requirements and guidelines applicable to the project, including the presence of drops and the location of the panels within the slab system.