Final answer:
A composite transposon is composed of chromosomal DNA flanked by two IS elements of the same type, with the IS elements containing inverted repeats and the transposition process generating direct repeats of target DNA at the insertion site.
Step-by-step explanation:
The structure of a composite transposon, also known as a Tn element, is a segment of DNA consisting of a central region that often contains genes conferring antibiotic resistance, flanked by two insertion sequences (IS elements) of the same type on each side. These IS elements include transposase and resolvase genes necessary for the mobility of the transposon. At each end of the IS elements are inverted repeats, and when these IS elements are inserted into genomic or plasmid DNA, the IS sequence itself is flanked by direct repeats of host genome or plasmid DNA that result from the mechanism of transposition.
When composite transposons engage in transposition, the transposase enzyme, which they encode, facilitates recombination at the inverted repeats and inserts the transposon into a new location in the DNA. This transposition process also results in the formation of direct repeats of the target DNA at the site of insertion. Thus, the correct description of the structure of a composite transposon is d) Chromosomal DNA flanked by two IS elements of the same type.