Question is in the attached photo. Pls help.

The answer for #3 is c but i don't know why

The table describes how some substances were formed substance 19 description formed by boiling pure water formed by combining three hydrogen atoms to every nitrogen atom formed by adding 5 g of sugar to 1 l of water formed by compressing carbon under high pressure based on the given descriptions, which substance is most likely a mixture?

The number to the right of an element's symbol (ex. c-12) identifies the of an isotope.