Smoking is injurious to health. Identify the word class of the underlined word 'Smoking'.

In the sentence 'Smoking is injurious to health', the word 'Smoking' is the subject of the verb 'is'. When an -ing form of a verb functions as a noun (here as the subject of the sentence), it is called a gerund. It is not a participle (which would modify a noun) nor an infinitive (which would use 'to'). Hence the correct answer is Gerund.