Why do you think plasmids and bacterial cells are such important tools in genetic engineering?

Plasmids are extracellular DNA they can be present in cells but in genetic engineering you can construct them specifically add markers and restriction sites .Once constructed add the restriction enzyme which recognizes these specific restriction sites cuts it open and then you bind your gene of interest to it stick it back. 
So basically say your gene of interest is a blue rubber band and your plasmid is a red rubber band. Your goal is to introduce that blue band to your red band.The restriction enzyme acts like scissors cuts the red band open and maybe gives it a weird shape and then gives the same cuts to your blue band so it fits into your red band.Then you have other enzymes like ligase which basically sticks the whole think together. 

The correct answer is d