Explanation:

Thallophyta is a group of plants that includes algae and fungi. These plants do not have well-differentiated structures like roots, stems, and leaves. They are simple and lack complex reproductive structures like flowers and seeds. Instead, they reproduce through various methods such as asexual reproduction and sexual reproduction.

In thallophyta, the formation of an embryo does not occur due to zygotic meiosis. Let's understand this in detail:

Zygotic meiosis in thallophyta:
Zygotic meiosis is a type of meiosis that occurs in the life cycle of certain organisms, including plants and algae. In this process, meiosis occurs immediately after the formation of a zygote, which is the result of the fusion of two gametes (sex cells).

The life cycle of thallophyta typically involves alternation of generations, where there are two distinct phases: the haploid (gametophyte) phase and the diploid (sporophyte) phase. In the haploid phase, gametes are produced through mitosis. These gametes fuse to form a diploid zygote in the process of sexual reproduction.

In thallophyta, after the formation of the zygote, zygotic meiosis takes place. This means that meiosis occurs directly in the zygote, resulting in the formation of haploid cells. These haploid cells then develop into the gametophyte generation, which produces gametes through mitosis.

Why embryo is not formed in thallophyta due to zygotic meiosis:
The formation of an embryo involves the fusion of male and female gametes and subsequent development. In thallophyta, the zygote directly undergoes meiosis, resulting in the formation of haploid cells, without any development into an embryo.

Since an embryo is not formed, thallophyta lack structures like seeds and fruits, which are characteristic of higher plants (like gymnosperms and angiosperms) that undergo embryogenesis.

Hence, the correct answer is option 'A' - Zygotic meiosis.