Nelson is Argyle's captain
However, Barnsley appear to have moved first for the 23-year-old Plymouth Argyle defender ahead of their Championship return next term.
Nelson, who captained the Pilgrims to the final of the League Two play-offs last season, is out of contract at Home Park but has been offered a new deal.
The chance to move up two divisions will be hard for Nelson to ignore, however, while a move to Oakwell would see him reunite with former Argyle midfielder Connor Hourihane.
Barnsley's Hourihane is another former Argyle skipper
Born in Newcastle-under-Lyme, the former Stoke City trainee has played nearly 250 games for the Pilgrims since 2010.
Rangers manager Mark Warburton admitted via the club website on Wednesday that he is keen to add another centre-back to his ranks, following Clint Hill's capture last week.
Mariappa, a former colleague of Warburton at Watford, was reported by the Daily Record to have held talks with the Scottish giants following his release by Crystal Palace.
Mariappa and Warburton go way back