What is the participial phrase in the sentence below?
instructed with a teacher's care, the students increased their knowledge of the civil war.
a. increase their knowledge
b. the students increased
c. instructed with a teacher's care
d. of civil war

The participial phrase in the sentence is C. INSTRUCTED WITH A TEACHER'S CARE.

Participial phrase always starts with a participle. The participle may be present or past participle. Present participle ends with -ing. Past participles end with -d or -ed. 

In the above sentence, the participial phrase started with a past participle, Instructed. Always remember, participial phrases serve as adjectives in the sentence. It modifies students in the sentence.