The functions of sieve tubes are controlled by
a)	Cytoplasm of sieve tube cells	b)	Nucleus of sieve tube cells
c)	Nucleus of companion cells	d)	Cytoplasm of companion cells

(c)

Nucleus of companion cells.

Sieve tube elements are long, tube-like structures, arranged longitudinally and are associated with the companion cells. Their end walls are perforated in a sieve-like manner to form the sieve plates. A mature sieve elements possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus. Functions of sieve tubes are controlled by the nucleus of companion cells