Hey there! I'm happy to help!
The trophic levels form a pyramid. The base level are the producers (plants), which are the most prevalent on the Earth.
Then, you have primary consumers (herbivores) which eat the plants. There are less of them because when something is consumed, only 10% of its energy is gained by the consumer, so one herbivore would have to eat 10 plants to get a viable amount of energy. If there were one herbivore for every plant, there wouldn't be enough plants for them all to live, so the herbivores would starve. That is why there are less, and this makes a pyramid.
After that, you have the secondary, tertiary, etc. consumers (carnivores) which eat the herbivores. There are less of these as the others as well. Depending on the food chain, you could have three, five, or many more levels of consumers, each group getting smaller and smaller until you reach the very top, so it depends on the ecosystem.
Although, this isn't diagrammed, decomposers are an important part of an ecosystem and this cycle of energy. If a lion eats a zebra and nothing eats the lion, where does the energy go? Decomposers (fungi, bacteria, etc.) take the remains of an animal and they decompose it, restoring the nutrients and energy back into the soil, which then allows for more plants to grow, and then the cycle continues!
So, if we look at our situation, we see that a flower is a producer as it is a plant that uses sunlight to grow. A grasshopper is a primary consumer as it eats the plant; it is an herbivore. A mouse eats the grasshopper, so it is an omnivore/carnivore and it is a secondary consumer. The last thing is a toadstool, which is a mushroom, and these are decomposers.
Flower - Producer
Grasshopper - Primary Consumer
Mouse - Secondary Consumer
Toadstool - Decomposer
So, if we start at our first trophic level (producers), our food chain will be D. flower, grasshopper, mouse, toadstool.
Have a wonderful day and keep on learning! :D
