Which characteristic best indicates that the organism in question is a fungus not a plant

There are quite a few differences that could count for this between plants and fungi. The first one would be the best characteristic:

1. Plants can make their own food via photosynthesis; fungi are obligated to get their food elsewhere so they can either be parasites and live off another living thing or they decompose matter to gain their energy.

2. So..fungi don't have chlorophyll...but plants do!

3. Plants reproduce through pollen and seeds - fungi make spores

4. Plant cell walls have cellulose whereas fungal cell walls have chitin.

5. Plants have roots, stems and leaves; fungi produce filaments which attach to a host (think of fungi you see growing on trees in the forest!)

All fungi are heterotrophic, which means that they get the energy they need to live from other organisms.

explanation:

the universal gravitation equation. since the gravitational force is directly proportional to the mass of both interacting objects, more massive objects will attract each other with a greater gravitational force. if the mass of one of the objects is doubled, then the force of gravity between them is doubled.