Final answer:
None of the given options form a term for a very young insect by adding just one letter. 'Pioneer species' are the first to colonize new environments, like land formed from volcanic lava.
Step-by-step explanation:
If you add a letter to a word for a product of Mount Etna, what term describing a very young insect is formed? The product of Mount Etna is lava, and by adding the letter "G" at the beginning of this word, it forms the term "glava", which does not relate to a very young insect. Hence, none of the given options (a) Lava (b) Maggot (c) Pupa (d) Eruption form a term for a very young insect by adding just one letter. Therefore, it seems there might be a misunderstanding in how the question was framed.
As for the second question provided as reference, the first species to live on new land, such as that formed from volcanic lava, are called d. pioneer species. These organisms are typically hardy species that are the first to colonize barren environments or ecosystems, starting a chain of ecological succession that ultimately leads to a more biodiverse steady-state ecosystem.