Which set of chemical elements listed below includes the most abundant elements found in living organisms, forming the core group that makes up the majority of biological matter?

Introduction / Context:
Biology and general science often require students to know which elements are most common in living organisms. These elements form the basic building blocks of biomolecules such as proteins, carbohydrates, lipids, and nucleic acids. This question tests recognition of that core group of elements, sometimes remembered using the mnemonic term CHNOPS, which stands for carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.

Given Data / Assumptions:

• The question asks for the most abundant elements in living organisms.
• All options are lists of six elements each.
• Only one option contains the classic set of biological building block elements.
• The goal is to choose the list that matches CHNOPS or its equivalent.

Concept / Approach:
Living matter is composed primarily of a small group of non metal elements. Carbon forms the backbone of organic molecules. Hydrogen and oxygen appear prominently in water and many organic compounds. Nitrogen is crucial in amino acids and nucleotides. Phosphorus appears in ATP and nucleic acids, and sulfur is present in some amino acids such as cysteine and methionine. Therefore, the correct answer has to include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur together in one list.

Step-by-Step Solution:
Step 1: Recall the standard list of major biological elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Step 2: Compare this list with each option one by one. Step 3: Option A includes oxygen, nitrogen, carbon, hydrogen, sulfur, and phosphorus, which exactly matches CHNOPS. Step 4: Option B replaces key non metals with metals such as potassium, iron, and magnesium, which are important but not the most abundant primary elements. Step 5: Options C and D also include metals like calcium, potassium, and sodium, which again are important but not the core group. Therefore, A is the only correct choice.

Verification / Alternative check:
A quick mental verification is to think about which elements are present in most biomolecules. For example, glucose contains carbon, hydrogen, and oxygen. Proteins contain carbon, hydrogen, oxygen, nitrogen, and often sulfur. DNA and RNA contain carbon, hydrogen, oxygen, nitrogen, and phosphorus. This pattern confirms that the CHNOPS group is central. The presence of metals like sodium or calcium is significant but they are not as universally abundant as the CHNOPS elements in the structure of biomolecules, confirming that option A is correct.

Why Other Options Are Wrong:
Option B lists potassium, iron, and magnesium, which are important micronutrients but do not represent the majority of atoms in typical cells. Option C includes calcium and potassium, again emphasising metals rather than the key non metals. Option D mentions sodium and calcium, which play roles in nerve conduction and structural support but are not the main elements forming organic molecules. Hence, all three of these lists omit at least one essential CHNOPS element and add less abundant metals instead.

Common Pitfalls:
A frequent mistake is to confuse the concept of abundance with importance. Calcium and sodium are very important physiologically but not as abundant in organic structures as carbon or hydrogen. Another pitfall is forgetting sulfur or phosphorus when recalling the CHNOPS set, which might cause confusion with a partially correct option. Memorising the CHNOPS mnemonic and understanding where each element appears in biological molecules helps avoid these errors.

Final Answer:
The most abundant elements in living organisms are oxygen, nitrogen, carbon, hydrogen, sulfur, and phosphorus.