Choose the most appropriate word to fill in the blank in the given se…

The correct answer is Option (4) → that

So ... that (cause and result):

• The pattern so + adjective/adverb + that + clause shows a cause and its result.
• The first part gives the degree ("so hot"); the clause after that gives the result ("I can't drink it").
• This sentence: Clue word: "so" ("so hot") calls for "that" before the result clause. The coffee is very hot, and the result is that the speaker cannot drink it, so the blank needs that.
• Complete sentence: The coffee is so hot that I can't drink it.
  Example: He was so tired that he fell asleep at once.

Why the Other Options are Incorrect?

Option 1: however — Incorrect. However is a linking adverb of contrast (but still); it cannot complete the "so ... that" pattern.
Example: The plan was good; however, it was too costly.

Option 2: but — Incorrect. But joins two contrasting ideas; there is no contrast between "so hot" and "I can't drink it".
Example: He is poor but honest.

Option 3: yet — Incorrect. Yet also shows contrast (but in spite of that); "so hot yet I can't drink it" makes no sense.
Example: The task was hard, yet she finished it on time.

Contrast: "so ... that" joins a degree to its result; "however", "but" and "yet" show contrast.