In which condition can a vector give higher DNA yield in a host?

Correct answer: It has a high copy number

The governing principle is plasmid copy number. Copy number means the average number of vector molecules maintained in one host cell. If a compatible plasmid has a high copy number, each transformed cell can contain many copies of the vector and, consequently, many copies of the inserted DNA. After growing a large population of such cells, the total recovered DNA is generally higher, provided that the insert remains stable and the host tolerates the plasmid. Option C is correct. A low copy number usually gives less DNA, absence of ori prevents normal plasmid replication, and absence of a selectable marker affects identification rather than directly increasing yield.