In animals, juvenile phase is followed by which phase ? 

Reproductive phase

The correct answer is Option (4) -Reproductive phase

All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. That period of growth is called the juvenile phase. It is known as vegetative phase in plants. This phase is of variable durations in different organisms.

The end of juvenile/vegetative phase which marks the beginning of the reproductive phase can be seen easily in the higher plants when they come to flower. 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.

• In animals, the juvenile phase (pre-reproductive stage) is the period of growth and development before an organism becomes sexually mature.
• This phase is followed by the reproductive phase, during which the organism attains sexual maturity and is capable of reproduction.
• The reproductive phase varies in duration depending on species, environment, and genetics.

Why the other options are incorrect?

Old age – Incorrect, because old age (senescence) occurs after the reproductive phase, not immediately after the juvenile phase.

Vegetative phase – Incorrect, as the vegetative phase refers to growth in plants, not animals.

Senescent phase – Incorrect, because the senescent (aging) phase occurs after reproduction, not immediately after the juvenile stage.