In the term trace element, the adjective trace means that

a. the elemental is required in very small amount.

b. the element can be used as a label to trace atoms through an organism's metabolism.

c. the elemnt is very rare oon Earth.

d. the element enhances health but is not essential for the organism's long-term survival.

The elemental is required in very small amount. (Ans. A)

Explanation:

Trace element is also known as micro-nutrient. It is also defined as any chemical element required by living organisms in a minute or small amounts which is usually part of the vital enzyme (cells produced by catalytic protein).

Exact needs of trace elements vary among species, like commonly required plant trace elements are cooper, zinc, manganese, boron, and molybdenum. Animals commonly required iodine, manganese, and cobalt.

Absence of necessary plant trace elements required by plants in the soil causes deficiency disease, lack of animal trace elements used by animals in the soil may not harm plants, but, animals feeding on those plants develop their deficiency disease.

So, the adjective trace means that the elemental is required in a very small amount.