Select the most appropriate option that can substitute the highlighted segment in the given sentence.
The students were boring by the lecture delivered on environmental issues.

Correct option is D

The most appropriate option that can substitute the highlighted segment in the given sentence is (d) the students were bored.
 "Boring" is an adjective, while "bored" is the past participle form of the verb "bore."
 In this context, you want to describe the state of the students, indicating that they found the lecture uninteresting or dull.
 Therefore, you need the past participle form "bored" to convey this meaning.