) moves to the beginning of the next sentence, therefore d) deletes (from the cursor) before the start of the next sentence. Vim discovers sentences using . , which means dot + space. This means that d) will have some problems if your cursor is on the dot or space that borders the two sentences, and will only delete up to the first character of the next sentence (that is, it will remove either a space or a period and a space, which is almost never desired). das will work as you probably expect, removing the sentence and delimiter (dot + space).
If you specifically want to move (and delete) the last character in a sentence, it will be harder according to this vi.SE answer :
Alejandro
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