What does ‘d’ represent in the figure?

A segment of DNA undergoes transcription .The segment of DNA which undergoes transcription forms transcription unit. In transcription unit  promotor situated at 5' end of the coding strand ,terminator situated at 3' end of the coding strand. The  strand which is showing polarity of 3'--5'  is template strand and the strand which is showing polarity of 5'--3' is coding strand.