Which feature makes a cloning vector useful for selection?

Correct answer: Selectable marker

The governing concept is selection of transformed host cells. A selectable marker is a gene carried by the vector that gives cells containing the vector a detectable survival or growth advantage under a defined condition. For example, an antibiotic-resistance marker can allow transformed bacterial cells to grow on medium containing that antibiotic, whereas cells without the vector are inhibited. Option A is correct because the marker distinguishes vector-containing cells from non-transformed cells. A circular shape may be common in plasmids but is not a selection mechanism. Gel colour and a fat layer neither identify transformants nor control their growth.