Which of the following causes inbreeding depression?

The correct answer is Option (1) → Continued self-pollination

Most flowering plants bear bisexual flowers, so pollen can easily land on the stigma of the same flower. When a plant is self-pollinated generation after generation, its vigour and fertility fall. This fall is called inbreeding depression. To avoid it, plants have outbreeding devices that discourage self-pollination.

Option (1) Continued self-pollination — Correct. Continued self-pollination is the cause the book gives for inbreeding depression. Repeated selfing keeps mixing the same set of genes, so the harmful effects build up over the generations.

Option (2) No pollination — Incorrect. Without pollination no pollen reaches the stigma, so there is no selfing at all. The book links inbreeding depression to continued self-pollination, not to a lack of pollination.

Option (3) Continued cross pollination — Incorrect. Cross-pollination brings pollen from another plant. This is what the outbreeding devices encourage, because it prevents inbreeding depression rather than causing it.

NCERT Extract:
Pollination – “… Outbreeding Devices : Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to come in contact with the stigma of the same flower. Continued self-pollination result in inbreeding depression. Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination.”