What is the main reason for offsetting the execution of the two cores in a lockstep CPU by a few clock cycles, or placing the two cores apart on the chip?

Answer: A. If the same cause, such as supply noise or radiation, produces the same error in both cores at the same time, the comparison cannot detect it, so this makes identical errors less likely

Lockstep detects faults through differences between the two cores' results, so it cannot detect identical errors produced in both by the same cause. Such failures are called common cause failures. Momentary supply voltage drops or clock disturbances affect both cores simultaneously, so delaying one core's execution by, for example, 2 clock cycles and realigning the results for comparison makes it less likely that the same error occurs at the same timing. Placing the cores apart likewise keeps localized causes from affecting both.