This is not part of apoplast

The correct option is "sieve tubes".

To understand why this is the correct answer, let's break down the concepts of plant transport pathways:
In plants, water and solute transport occurs through two distinct pathways: the apoplast and the symplast.

1. The Apoplast Pathway:
The apoplast is the non-living network of cell walls and intercellular spaces throughout the plant. Transport through the apoplast is passive and occurs purely by diffusion and capillary action, as there are no selectively permeable membranes to cross. The main components of the apoplastic pathway include:
- Cell walls: Outer non-living boundaries of plant cells.
- Intercellular spaces: Air spaces between adjacent cells.
- Vessels (and Tracheids): Non-living, hollow xylem elements that conduct water.

2. The Symplast Pathway:
The symplast is the continuous system of living cytoplasm connected by plasmodesmata (microscopic channels crossing the cell walls). Transport through the symplast is active/passive regulated transport across plasma membranes and through living protoplasts.

Why Sieve Tubes are not part of the Apoplast:
Sieve tubes are specialized living cells (sieve tube elements) in the phloem tissue responsible for transporting organic nutrients (like sucrose). Although they lack nuclei at maturity, they possess a living plasma membrane and cytoplasm. Because sieve tubes are living cellular components containing cytoplasm and are bounded by active plasma membranes, they are part of the symplastic pathway, not the apoplast.