Choose the most appropriate option to fill in the blank in the given…

The correct answer is Option (3) → too much

"Too much", "much too" and "only too":

• Too much + uncountable noun = more than is acceptable: too much pain, too much noise.
• Much too + adjective or adverb = far too: much too painful, much too late.
• Only too + adjective = very, completely: only too glad to help.
• This sentence: "pain" is an uncountable noun, so too much fits.
• Complete sentence: His behaviour gave me too much pain.
  Example: Do not add too much salt to the curry.

Why the Other Options are Incorrect?

Option 1: only too — Incorrect. "Only too" goes before an adjective, not before a noun such as "pain".
Example: I am only too happy to help.

Option 2: much too — Incorrect. "Much too" goes before an adjective or adverb ("much too painful"), not before a noun.
Example: This shirt is much too tight.

Option 4: to much — Incorrect. "To" is a preposition; the word meaning "more than enough" is spelt too.