The correct option is C was
In order to find out the correct answer, let's first understand what linking verbs are. Linking verbs are the words that link the subject to a noun or it's adjective. For example: I am a girl. Here, 'I' is linked to the word 'girl' by the word 'is', hence 'is' is the linking verb here.
Here, we have to identify the linking verb from the given sentence.
NOTE: Verbs are action words and linking verbs explain the subject's state of being.
Let's have a look at the options:
Option A - 'Rajni thought that...', here, 'thought' is an ordinary verb, as it simply explains subject's action. It doesn't link the subject with its state of being. Hence, option A is incorrect.
Option C - 'Very' is used for emphasis, as an adverb or an adjective. It's not a verb. Hence, option C is incorrect.
Option D - 'Good' is an adjective as it explains the state of the subject. Adjectives are used to explain more about nouns. It's not a linking verb. Hence, option D is incorrect.
Option B - '...the college was very good', here, 'was' links the subject 'the college' to its state of being 'very good'. Hence, we can say that 'was' is a linking verb as it links the subject and its adjective. Thus, option B is correct.