In grass families, the cotyledon is called as:

The correct answer is Option (4) → Scutellum

A monocot embryo, as in grasses, has only one cotyledon, called the scutellum. It lies to one side of the embryonal axis.

Option (1) Coleoptile — Incorrect. It is the sheath around the shoot apex (plumule) of the grass embryo.

Option (2) Coleorhiza — Incorrect. It is the sheath around the radicle and root cap.

Option (3) Thalamus — Incorrect. The thalamus is part of the flower (the receptacle), not of the embryo.

Option (4) Scutellum — Correct. The embryo of grasses has one cotyledon, called the scutellum. It lies towards one side (lateral) of the embryonal axis.

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NCERT Extract:
Embryo – “… Embryos of monocotyledons (Figure 1.14 b) possess only one cotyledon. In the grass family the cotyledon is called scutellum that is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called coleorrhiza. The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.”