Final answer:
The mode of nutrition is the least helpful characteristic for distinguishing between plants, animals, and fungi.
Step-by-step explanation:
The least helpful characteristic for distinguishing between plants, animals, and fungi would be mode of nutrition. This is because plants perform photosynthesis, animals are heterotrophs, and fungi have a variety of nutrition modes including saprophytic, parasitic, and mutualistic. Therefore, the mode of nutrition can vary within each kingdom and does not provide a clear distinction between them.