This cup has the _____ sugar of all.

The task is to choose the correct word to complete the sentence. The sentence is about the amount of sugar in a cup, and it's a superlative comparison (indicated by "the" and "of all"). We need to select the superlative form of "little".

The comparative form of "little" is "less", and the superlative form is "least". Since the sentence uses "the" and "of all", it requires the superlative form.

1. less: This is the comparative form, used for comparing two things. Example: "This cup has less sugar than that one."

2. least: This is the superlative form, used for comparing three or more things, or when referring to the lowest degree of something. Example: "This cup has the least sugar of all the cups."

3. little: This is the base form (or an adverb), used in affirmative statements without comparison. Example: "This cup has little sugar."

Therefore, the correct word to complete the sentence is "least".

Ответ: least