Hint: Explant is a piece of organ, tissue, etc. which can be transferred into a nutrient medium for preferential growth. Its characteristic features include, high meristematic activity and least susceptibility to pathogens.
Complete answer: During artificial tissue/ cell culture, the developing plant may be prone to several diseases. To prevent that, the explant is taken from a safe plant part that is less likely to be infectious.
1. The explant should also have high meristematic activity in order to get a desired culture within the stipulated time.
2. High meristematic activity is seen in apical meristem, axillary bud and leaf primordial. The meristematic tissue is a part of the procambium, they consist of several undifferentiated cells known as meristematic cells.
3. Embryos cannot be used as explants to obtain such pathogen free plants because they are likely to be affected by the pathogens.
4. Developing embryos are in contact with the external environment through pores and the pollen tube.
5. Apical bud is the most efficient in terms of meristematic activity.
The correct answer is, option 1: Embryo
Note: Apical meristem is the growth region in plants found within the root tips and the tips of the new shoots and leaves, they are influenced by auxins.
Leaf primordial is primarily the young leaves that have formed recently from the tip of the shoot, formed by the apical meristem.
Axillary bud is the embryonic or organogenic shoot located in the axil of a leaf.