Missing Letters, Prefix and Suffix

 The correct answer is "ba" because it forms the word "aback" as in "he was taken aback by the surprise gift from his wife."

The word is abuse. Adding ac to ab results in abuse.

The word is about. Adding bo to ab results in about.

Preheat is a valid word formed by adding the prefix pre to heat.

Unhappy is the standard antonym of happy, formed by adding the prefix un.

The word is abduct. Adding du to ab results in abduct.

The word is aberration. Adding er to ab results in aberration.

Unaware is the standard antonym of aware, formed by adding the prefix un.

Uncomfortable is the standard antonym of comfortable, formed by adding the prefix un.

Abnormal is the standard antonym of normal, formed by adding the prefix ab.

Unturn is a recognized, though less common, verb meaning to reverse a turn.

Unsent is the standard antonym of sent, formed by adding the prefix un.

Nonsense is a common noun formed by adding the prefix non to sense.

The word is abbey. Adding be to ab results in abbey.

Cheerful is a common adjective formed by adding the suffix ful to cheer.

Careless is a common adjective formed by adding the suffix less to care.

The word is abase. Adding as to ab results in abase.

Unfold is a common verb formed by adding the prefix un to fold.

Unhealthy is the standard antonym of healthy, formed by adding the prefix un.

Doubtful is a common adjective formed by adding the suffix ful to doubt.

Thankless is a common adjective formed by adding the suffix less to thank.

Hopeless is a common adjective formed by adding the suffix less to hope.

The word is abandon. Adding on to aband results in abandon.

The word is abandoned. Adding do to aban results in abandoned.

A suffix is a letter or group of letters added to the end of a word to change its meaning. Adding -ful to the root word 'use' creates the adjective 'useful'.

The suffix -able is added to the verb 'read' to form the adjective 'readable', which means capable of being read.

None of the provided options (Sweeting, Sweetless, Sweetful) are standard English words derived from 'sweet' in the context of common suffixes. Therefore, 'None of these' is the correct choice.

The word is 'abdicate', which means to renounce one's throne or responsibility. The missing letters are 'di'.