Which of the following is true for compounds? a) They can be created by chemical reactions.
b) They can be separated into their component elements through physical means.
c) They all have the same properties regardless of their elemental composition.
d) They can consist of no more than two types of elements.

The correct option is that the compounds are created by chemical reactions.

Explanation:  

Option 1: Elements combine chemically in a specific ratio to form compounds. They can't be separated by physical methods like filtration. For example,  is made up 2 H-atoms and 1 O-atom. When we filter water through normal filtration, we do not obtain H-atom and O-atom separately, but we get combined form of hydrogen and oxygen atom that is .

Option 2: Compounds are formed when two or more elements combine chemically.

Option 3: Compounds can have more than two types of elements. For example, In , 3 elements are present - Calcium(Ca), Carbon(C), Oxygen(O).

Option 4: Compounds formed have different properties from one another. There can be difference in physical properties as well as chemical properties. For example,  (water) and  (calcium carbonate) are two compounds. Water is present in liquid state and calcium carbonate is present in solid state. Hence, it showed that all the compounds cannot have same properties

- 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

increased stability in the completion of fluorine's outer shell.