Question

(in February), Jonathan decided to send in his application.

Answer 1

Here all the answers:
1) Some youngsters under the age of four
Adjectival
2) encourage them with words of praise
Adverbial
3) The older siblings of these young children
Adjectival
4) and swing from one bar to the next
Adverbial
5) Seaweed sometimes grows to 200 feet
Prepositional
6) In February, Jonathan decided (your question)
Prepositional
7) Joan, my oldest aunt,
Appositive
8) studied the list of possible candidates
Prepositional

Answer 2

The answer is prepositional.