Bamboo species exhibit unusual flowering phenomenon, because it flowers:

only once in their life time.

The correct answer is Option (2) → only once in their life time.

Plants –the annual and biennial types, show clear cut vegetative, reproductive and senescent phases, but in the perennial species it is very difficult to clearly define these phases. A few plants exhibit unusual flowering phenomenon; some of them such as bamboo species flower only once in their life time, generally after 50-100 years, produce large number of fruits and die. Another plant, Strobilanthus kunthiana (neelakuranji), flowers once in 12 years.