The blank requires a verb to denote the action that is done by the police with respect to the people involved in the theft. Among the given options, 'fortunate' is an adjective and 'eagerly' is an adverb and so they are negated. 'Attended' which means be present at' and 'gambled' which means 'play games of chance for money; bet' are verbs that do not match the context of the given sentence and so they are negated. Option (c) 'arrested' means 'seize (someone) by legal authority and take them into custody' is the most appropriate verb that matches the context of the sentence and can denote the action that is done by the police to the culprits.
The complete sentence is: The police arrested the culprits who were involved in the theft.
Hence, the correct answer is option (c).