Which types of elements are primarily found in living organisms?

Living organisms are primarily composed of elements that play critical roles in biological processes. The correct choice highlights the presence of small non-metals and some metals, which are fundamental to life.

Small non-metals such as carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur are essential building blocks of biological molecules. For instance, carbon serves as the backbone for organic molecules, while oxygen and hydrogen are vital components of water, which is crucial for life. Nitrogen is a key element in amino acids and nucleotides, making it critical for protein synthesis and genetic material.

Additionally, some metals, though present in trace amounts, are vital for various biochemical functions. For example, iron is essential for oxygen transport in hemoglobin, magnesium plays a crucial role in enzyme function, and zinc is involved in numerous biochemical reactions as a cofactor.

The other options presented do not appropriately represent the elemental composition of living organisms. Heavy metals can be toxic and are not integral to life processes. Larger metals are typically less abundant and are not essential in the way that smaller non-metals and certain trace metals are. Radionuclides, while interesting in some biological contexts, are not fundamental to the composition or functioning of living organisms. Thus, the answer correctly identifies the