Question

life depends on this important element
○ carbon
○ boron
○ lithium
○ oxygen

compounds are
○ formed by two or more different types of elements which are united chemically in an unfixed proportion.
○ formed by two or more different types of elements which are united chemically in a fixed proportion.
○ formed by two or more different types of elements which are united artificially in a fixed proportion.
○ formed by four or more different types of elements which are united chemically in a fixed proportion.

protons are
○ negative
○ chemical
○ positive
○ neutral

Explanation:

First Question: "Life depends on this important element"

Carbon is the basis of organic life as it can form complex molecules (like proteins, carbohydrates, nucleic acids) essential for life processes. Boron, Lithium have limited roles in general life support, and Oxygen is important but carbon is fundamental for organic life's molecular structure.

By definition, a compound is a substance formed when two or more different elements chemically bond in a fixed proportion (e.g., $H_2O$ has 2 H and 1 O always). The first option says unfixed proportion (wrong, that's mixtures), third says artificial (compounds can form naturally), fourth says four or more (compounds can have 2 elements, e.g., $NaCl$).

In atomic structure, protons are subatomic particles with a positive electric charge. Electrons are negative, neutrons are neutral, and "chemical" is not a charge description.

Answer:

A. Carbon

Second Question: "Compounds are"