When a vegetative cell forms an embryo without fertilization, it is called as
(a) Apospory
(b) Apogamy
(c) Diploid polyembryony
(d) Adventive polyembryony

Hint: Formation embryo from vegetative cells without fertilization is common in citrus and mangifera. In citrus, it results from the proliferation of nucellar cells. It is the development of embryos from the extra embryo sac tissue, which includes maternal sporophytic tissues such as nucleus and integuments.

Complete answer:
Adventive polyembryony is the development of embryos from the extra- embryo sac tissue, lying outside the embryo sac. such tissues include maternal sporophytic tissues, such as nucellus and integuments. Adventive embryos are common in citrus and mangifera. In citrus, adventive polyembryony results from the proliferation of nucellar cells. The inception of nucellar embryos takes place outside the embryo sac. But they are gradually pushed into the embryo sac cavity where they differentiate into mature embryos. The presence of endosperm probably promotes the development of adventive embryos at the Micropylar end, but it represses their development at the Chalazal end. Polyembryony is the occurrence of more than one embryo in a seed whereas adventive polyembryony is the formation of an embryo directly from the diploid sporophytic cell or vegetative cell. It takes place without the involvement of processes like meiosis and mitosis. For example, in citrus 40 embryos are formed out of which one embryo is formed by fusion while others are adventive.

Additional Information:
- Apogamy is the development of 1 n sporophytes without gametes and syngamy from vegetative cells of the gametophyte.
- Apospory is the development of 2 n gametophytes, without meiosis and spores, from vegetative, or non- reproductive, cells of the sporophyte.
- Diploid polyembryony occurs when a diploid embryo is formed by the polyembryony without normal fusion and meiosis.
So, the correct answer is '(d) Adventive polyembryony'.

Note:
- Polyembryony is the development of more than one embryo in an ovule. In other words, it is the formation of two or more embryos in a seed. In normal development, if additional or supernumerary embryos are formed, they will degenerate sooner or later.
- However, in many plants, such as citrus, Mangifera, etc. the extra embryos also may persist. Polyembryony has been classified in different ways by different authors.
- Some of the common types of polyembryony are True polyembryony, false polyembryony, cleavage polyembryony, adventive polyembryony, polyembryony in which multiple embryos are formed from the embryo sac cells, other than the egg cell.