Q.Plasmids are
a. circular protein molecules
b. required by bacteria
c. tiny bacteria
d. confer resistance to antibiotics

A vector (cloning vehicle) is a small, self-replicating DNA molecule used to carry and transport a DNA fragment into a host cell for propagation, splitting functionally into cloning vectors (which simply propagate an insert) and expression vectors (which additionally carry the signals needed to translate the insert into protein, e.g. pUC19). A workable vector must be small (under about 10 Kb), carry an origin of replication (ori), a selectable marker (typically antibiotic resistance) and a Multiple Cloning Site (MCS/polylinker) of unique restriction sites. The major named vector families are: plasmids (extrachromosomal, self-replicating circular bacterial DNA - pBR322, built by Bolivar and Rodriguez, being the classic 4361 bp example, and the tumour-inducing Ti plasmid of Agrobacterium tumefaciens being the key plant-biotechnology example); transposons ('jumping genes' that insert without sequence relationship to their target site); cosmids (plasmid + lambda-phage cos site hybrids); bacteriophage vectors (lambda phage, M13, carrying inserts up to ~25 Kb); phagemid vectors (plasmid + phage-derived origin, e.g. pBluescript SK); the Yeast Artificial Chromosome (YAC, circular in bacteria, linear in yeast); shuttle vectors (replicate in two different host species); and the Bacterial Artificial Chromosome (BAC, stable, cloning inserts up to ~300 Kb).