Which one of the following is a micro-element ?

Answer: Option A

In analytical chemistry, a trace element or microelement is an element in a sample that has an average concentration of less than 100 parts per million measured in atomic count or less than 100 micrograms per gram. These are those elements essential for plant growth which are needed in only very small (micro). They are: boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn).