The correct option is C our
In the line "That day I realized how a little change in .... attitude could make the world a much better place to live in," the speaker states a general truth which applies to everyone. By using 'our' the speaker means himself, the lady and everyone else in general. Hence, Option B is correct. Option A is incorrect because the line is meant to reflect on everyone and the speaker doesn't mean it in the first-person singular form. Option C is incorrect because the speaker does not refer to the lady specifically. Option D is incorrect because the speaker is not speaking in the third-person form.