The translocation of organic solutes in sieve-tube members is supported by:
A. Root pressure and transpirational pull
B. P-protein
C. Mass flow involving a carrier and ATP
D. Cytoplasmic streaming

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Hint: In flowering plants, the sieve tubes are the elongated cells that are living in the phloem. They are the conduits of food transport mostly sugar. These are arranged in an end-to-end manner to form a tubular structure called the sieve tube.

Complete answer: According to the mass flow hypothesis, the source provides the way to the transportation of organic solutes to sink and this is not completed by the help of metabolic energy.
Whereas according to the cytoplasmic streaming hypothesis, the organic solute transportation takes place by the combination of diffusion and cytoplasmic streaming. This was put forward by DeVries in 1885.
-P-protein has a defensive role against the insects who feed on phloem and sealings of damaged sieve tubes. Monocots and gymnosperms lack protein.
-Root pressure is caused by the active distribution of ions of mineral nutrients. A force is exerted that pushes the water upon the stem on a specific level is called Root pressure. The force that aids in drawing the water in an upward direction from roots to leaves is known to be as Transpirational pull. The negative pressure produced by the transpirational pull then exerts a force on the water particles pushing the direction of water to an upward direction or movement within the part of the plant.
Hence, the correct answer is option (C)-Mass flow involving a carrier and ATP.

Note: The reason behind the living nature of the sieve tube in phloem is they have living protoplasts only when they mature. An elongated, food-containing cell in phloem in angiosperms. They lack a nucleus and are companion cells dependent on certain functions.