a. The question is asking which organism provides energy to all the other organisms, directly or indirectly, so you should look at the first organism on the food web; the garden plants. The garden plants provide energy to the aphids, slugs, and snails, which then provides indirect energy to the other organisms.
b. The amount of food available to the slug population may increase if the aphid population decreases because there are less organisms consuming the garden plants, which means that there will be more food available to the slugs.
