The correct option is C worrying Gerunds are words that are formed with verbs but act as nouns. Gerunds also at times are seen to end in 'ing'. In this particular sentence, the word in the blank acts a noun thus becoming the subject of the sentence and also like a verb by taking up the object. This means that in the blank, we would need a gerund.
Option C: Worrying is a gerund as it plays a dual role
Option A and B: Worry is in the present tense. It is the wrong option
Option D: Worried is the past tense again. It is the wrong option