Final answer:
The best word to complete the sentence is 'nostalgia' as it accurately describes the sentimental yearning for the joy of past experiences, which is the emotion being expressed in the sentence.
Step-by-step explanation:
The sentence 'The nostalgia that I felt for my childhood made me suddenly burst into tears' is completed correctly with the word nostalgia. Nostalgia refers to a sentimental yearning for the happiness felt in a former place, time, or situation. It is often associated with fond, albeit bittersweet, memories of one's past, as reflected in the context of the sentence that suggests an emotional connection to childhood experiences.
The other options, like remorse, longing, and regret, convey different nuances of emotions. Remorse is a deep regret or guilt for a wrong committed, while longing is a yearning desire, and regret is a feeling of sadness, repentance, or disappointment over something that has happened or been done. Thus, the most appropriate choice that encapsulates a sense of tender reflection on the past is nostalgia, especially in relation to the fond memories implied by the context of the cry.