$(a)$ Promoter

Promoter is a region of $DNA$ that helps in initiating the process of transcription. It serves as the binding site for $RNA$ polymerase.

$(b)$ $t R N A$

$tRNA $ or transfer $RNA$ is a small $RNA$ that reads the genetic code present on $mRNA$. It carries specific amino acid to $mRNA$ on ribosome during translation of proteins.

$(c)$ Exons

Exons are coding sequences of $DNA$ in eukaryotes that transcribe for proteins.