The correct word is 'धीरे' (slowly) because it denotes the manner of the action. 'Speak slowly' is a meaningful adverbial usage.

'तक' here is used as a preposition/particle (sambandhbodhak) that connects with the noun to indicate a limit.

Avyaya is also called 'Avikari'. The literal meaning of Avyaya is- that which does not change (expend). Avikari means- that which does not have any distortion or change.

In option (1), 'hi'; in option (2), 'bhi'; and in option (4), 'to' particles are used.

'Carefully' (Dhyanpurvak) is an adverb of manner because it describes the manner or way in which the action is performed.

'Dheere-dheere' (slowly) is an adverb of manner.

Option (3) does not have any adverb. In other options, 'suddenly', 'carefully', and 'often' are adverbs.

'Bhi' here is a particle (Nipat), which is a type of indeclinable (Avyaya).

In option (3), 'bhi' is used as a particle.

In Hindi grammar, 'विरुद्धवाचक' is not strictly counted as a standard type of संबंधबोधक अव्यय (often it is classified as विरोधवाचक).

'Achanak' (suddenly) indicates the manner of the action.

In option (1), 'tak'; in option (2), 'bhi'; and in option (3), 'to' particles have been used.

Adjectives are mutable words (Vikari), whereas postpositions, conjunctions, and adverbs are indeclinable (Avyaya).

Interjections (vismayadibodhak) are not an integral part of the sentence; they are independent of the sentence.

The primary function of a conjunction is to connect two words, phrases, or clauses.

'Dheere-dheere' (slowly) indicates the manner of the action.

'Contemporary' can be used as an adjective, whereas the others are adverbs.

Words in which no change occurs according to usage are called Avikari or Avyaya words.

'चुप' is a word used to express contempt or disdain.

In options 1 and 2, 'के' is a preposition, and in option 3, 'के बिना' is a preposition. In option (4), 'पहले' is used as an adverb of time.