In which of the following plants vegetative reproduction takes place with the help of bulbils?

The correct option is C Agave
Bulbils are a small bulb-like structure which may be removed to form a new plant. Agave can sexually reproduce by seeds and vegetatively by aerial bulbils and ground level based shoots and rhizomes. Colocasia reproduces sexually by seeds and vegetatively by corms, tubers and root suckers. Zingiber reproduces vegetatively by aromatic articulated rhizomes. Vallisneria reproduces vegetatively through runners.

So, the correct answer is option C.