In the above mentioned sentence, the most appropriate word to be filled in the blank is Option C “kindly”, as it is grammatically the most suitable word given the nature of the sentence. The sentence describes the attitude of the teacher towards her naughty children, and with the given options, the correct form of ‘kind’ should be “kindly.” "kindly" modifies the verb "spoke" as it tells us the way in which the teacher spoke to the students.
The other options, “kind”, “kindness”, and “kindliness” are not suitable forms to be included in the sentence according to the situation provided, hence Option C is the correct answer.