Consider the above diagram of DNA molecules each 1000b long

Consider the above diagram of DNA molecules, each ~1000b long. Which molecule(s) could be a substrate for DNA polymerase III? C and D B only A and D A only A, C and D

Solution

D] A only

DNA polymerase III acts on the lagging strand fro the lagging strand synthesis and adds nucleotides on the 3\' OH of the primer.
Since B has a completely synthesised strand, it will not be the substrate. In D, primers have to be added to the leading strand, which is not the function of DNA pol III.