The correct answer is option 'D', kindly.

Explanation:
The word 'kindly' is an adverb that describes the manner in which the teacher spoke. It is used to convey politeness or gentleness in speech or action.

Here is a breakdown of the options:

a) kind: This is an adjective that describes the teacher, not the manner in which they spoke. While it may be true that the teacher is kind, it does not specifically address the way they spoke.

b) kinder: This is the comparative form of the adjective 'kind'. Similar to option a, it describes the teacher rather than their speech.

c) generously: This adverb describes the generosity of the teacher, not the manner in which they spoke. It implies that the teacher is generous in some aspect, but it does not specifically refer to their speech.

d) kindly: This adverb is the correct choice as it directly describes the manner in which the teacher spoke. It indicates that the teacher spoke in a polite and gentle manner.

In this context, the word 'kindly' is the most appropriate choice as it precisely conveys how the teacher spoke to the speaker. It indicates that the teacher used a polite and gentle tone while communicating with the speaker.

Using the word 'kindly' in the sentence helps to emphasize the respectful and considerate nature of the teacher's speech, which is the intended meaning of the sentence.