The correct option is A enucleated
Sieve tube cells are elongated living cells which are arranged longitudinally in association with companion cells. As they mature, they undergo a series of progressive changes that result in the breakdown and loss of nucleus, the tonoplast, Golgi bodies and ribosomes. At maturity, these cells have a functional plasma membrane and modified mitochondria, plastids and smooth endoplasmic reticulum, but lack nucleus. So they are enucleated.
The functions of sieve tubes are controled by the nucleus of the companion cell.