Atheory is a a) proposed explanation for an observation b) well-tested explanation for a broad set of observations c) summary of the results of many observations d) procedure used to test a proposed explanation

Option (b) is the correct answer.

Explanation:

A statement which states a scientifically acceptable general principle that is experimentally tested or verified is defined as a theory.

A theory is the sum of a number of observations which are tested in order to get the true result so that a statement can be framed.

Thus, we can conclude that out of the given options, a theory is a well-tested explanation for a broad set of observations.

- calcium has greater affinity to bond with electron receptors than hydrogen does and hence, it has the ability to replace hydrogen in most of its compounds.

- lithium has 2 valence electrons in the product because it can make only 1 bond with hydrogen. beryllium has 4 valence electrons in the product because it can only make 2 bonds. it has fewer electrons than neon and can react with electron donors. each hydrogen has 2 valence electrons, the same as helium.

- the covalent bond formed between carbon and hydrogen is polar because carbon is more electronegative than hydrogen and it attracts the bonded electron pair closer to itself, thus giving hydrogen a partial positive charge.

- a hydrogen bond is a relatively weak bond that hydrogen atoms make with the electronegative atoms nitrogen, oxygen or fluorine.

so the best choice is a.) 2 calcium (ca) atoms