Answer:
d. a plasmid used to transfer DNA into a living cell.
Explanation:
In molecular biology, it's very useful to use vectors to insert a DNA fragment of interest into a host cell.
In this technique, a DNA sequence of interest is isolated from a larger DNA molecule or even from a complete genome by cutting the DNA with restriction enzymes. At the same time, the restriction enzymes are used to cut open a plasmid, a circular molecule of DNA that can replicate independently and survive for a certain amount of time in the host cell (how much time, it depends on the cell). The fragment of interest is then linked to open plasmid, forming the vector. The vector can be introduced into a living cell, and the sequence of interest can be analyzed in different ways.
I've attached a diagram made by the University of Waikato that illustrates the procedure for creating a vector.
