Syncytium is formed by:

✅Explanation:
A syncytium is a multinucleate cell formed by the fusion of cells, often seen in the feeding sites of parasitic nematodes. In the case of cyst nematodes (e.g., Heterodera species), the syncytium forms in the cortical parenchyma of the plant root. This structure is critical for the nematode's feeding, allowing it to extract nutrients from the plant by fusing and modifying the plant cells at the feeding site.

📌Other Options Explanations:
(a) Root-knot nematodes in cortical parenchyma
Root-knot nematodes (Meloidogyne spp.) cause the formation of galls in the root cortex. They form root galls.
(d) Citrus nematodes in cortical parenchyma
Citrus nematodes (Tylenchulus semipenetrans) are typically endoparasitic and cause galls or lesions rather than syncytia in roots.