Seedless fruit can be obtained by treating the unpollinated ovaries with
(a) Colchicine
(b) Sucrose solution
(c) Hormones
(d) Pure lanolin

Hint: Production of seedless fruit through a single unfused gamete is parthenogenesis. In parthenogenesis commonly female gamete led to the production of fruit, it can take place naturally or artificially by using physical or chemical methods.

Complete answer:
Plant growth hormones are the chemical substance which regulates growth, differentiation, and development of the plant. Growth hormone-like auxin induces parthenogenesis by its application on unpollinated ovaries which leads to the production of seedless fruit. Auxin was first discovered by F.W. Went in coleoptile tip and it was the first plant hormone which was discovered. It is a weakly acidic growth hormone that promotes cell elongation at a concentration of less than 100 ppm.

Additional Information: Auxins are of two types: natural and synthetic. Natural auxins synthesized by plants are therefore also known as phytohormones. Indole3-acetic acid, indole-3-acetaldehyde, indole-3-acetonitrile, Indole-3-butyric acid, phenylacetic acid, and 4-chloro income acetic acid are natural auxin which synthesizes in shoot apices, leaf primordia, and developing seeds. These hormones move from one cell to another by diffusion. Synthetic auxin is 2,4-dichloro phenoxy acetic acid, 2,4,5-trichlorophenoxy acetic acid, Indole 3-butyric acid, and Naphthalene acetic acid. It moves in all directions inside the plant. Both these hormones help in the elongation of roots and shoot, a higher concentration of these hormones show an inhibitory effect on growth and also cause retardation. Another function shown by auxin is
Cell enlargement by solubilization of carbohydrates, loosening of microfibrils, and respiration which led to the enlargement of fruits.
Cell division in cambium takes place in the growing season from developing shoot buds.
Root growth and root initiation take place at a low concentration of 0.0001 ppm and concentration helps in shooting elongation but inhibits root growth.
Apical dominance by the development of lateral buds into branches and inhibits the growth of nearby lateral buds.
Inhibit the abscission of mature leaves and ripe fruits from the plant. It also cut off nutrients and water supply.
So, the correct option is ‘Hormones.’

Note: -Bioassay of auxin was performed in Avena coleoptile and cress root to test its activity in growth response in plant parts.
-Indole 3- butyric acid is both a natural and synthetic hormone that is used for high crop production and ornamental plants.