The correct spelling is option 'B' - Adultery.

Explanation:
Adultery is the correct spelling for the word that refers to the act of a married person engaging in a sexual relationship with someone other than their spouse. Let's break down the spelling and understand it further.

1. Root word: The root word is "adult". It refers to a person who has reached the age of maturity or adulthood.

2. Suffix: The suffix "-ery" is added to the root word to form the noun "adultery". This suffix is commonly used to form nouns that denote a state, condition, or quality.

Now, let's understand the meaning of the word "adultery":

Adultery refers to the act of a married person having sexual relations with someone other than their spouse. It is considered a breach of the marital vows and is generally frowned upon in many societies.

The word "adultery" is used to describe the specific act and is different from words like "infidelity" or "cheating" which are more general terms.

In conclusion, the correct spelling of the word is "adultery" (option 'B'). It is important to use the correct spelling in order to effectively communicate and convey the intended meaning.