Step 1: Understanding the Concept:
This question tests idiomatic knowledge and usage of adverbs in English. The phrase "know too [blank]" requires an adverb that describes the depth or extent of knowledge.
Step 2: Analyzing the Options:
• (A) "More" is a quantifier and does not fit the structure "know too more."
• (B) "Little" and (C) "few" act as quantifiers or adjectives. While "you know too little" is a grammatically correct sentence, it does not fit the context of identifying "who could have spilled the beans" based on long-term employment.
• (D) "Well" is an adverb. The idiom "know too well" is used to express that someone is very familiar with a situation or person.
Step 3: Detailed Reasoning:
In the context provided, the speaker is emphasizing that their long tenure has given them deep familiarity with the situation. The expression "know [someone/something] too well" is a standard English idiom meaning to have thorough knowledge or awareness. Thus, "you know too well who could have spilled the beans" is the logically and grammatically correct choice.
Step 4: Final Answer:
The correct word to fill the blank is "well."