Option (b) is the correct answer as the source of the complementary RNA for RNAi could be mobile genetic elements (transposons) that replicate via an RNA intermediate.

Option (c) is incorrect as autoradiography usually follows hybridisation.

Option (a) is incorrect because polymerase chain reaction is used to make copies of the DNA sample and does not need transposons.

Option (d) is incorrect because transposons are not required during gene sequencing.