Answer: The correct answer is 3) A segment of a DNA molecule that contains the instructions for producing a trait in an organism.
Gene is a segment of DNA molecule that contains instructions ( in the form of nucleotide sequences), which are required for producing a specific trait ( characteristic) of an organism.
For this, gene is first transcribed into mRNA ( messenger RNA), which is then translated to produce a particular protein. The protein thus formed gives a specific characteristic to the organism.
Thus, option 3) is the best description for a gene.