​In which of the following stages of Arabidopsis embryogenesis do the visible distinctions between the adaxial and abaxial tissues of the cotyledons become initially apparent?​

Correct option is C

Explanation-

In Arabidopsis embryogenesis, the development proceeds through several well-defined stages:
1.  Zygotic stage: Formation of the zygote. Only a single-celled embryo.
2.  Globular stage: Embryo is spherical, no visible polarity in cotyledons yet.
3.  Heart stage : First emergence of cotyledon primordia.
4.  Torpedo stage: Clear distinction appears between adaxial (upper) and abaxial (lower) sides of cotyledons.
5.  Mature stage: Fully formed cotyledons and other tissues. Adaxial-abaxial polarity is already established; this is not the initial stage of its appearance.

So, the  correct answer is option c - Torpedo stage.