Spelling and usage – which of the following is a correctly written compound word?

Introduction / Context:
This question tests your understanding of how English compound words are correctly written and hyphenated. Compound words can appear as open compounds (two separate words), closed compounds (one word), or hyphenated compounds. Exam questions often present several near correct spellings and ask you to pick the correctly formed compound according to standard usage.

Given Data / Assumptions:

Each option shows a phrase where two or more words may need to be combined or hyphenated.

We assume standard modern English spelling as used in school textbooks and formal writing.

Only one option should represent a well established compound spelling; the others should contain errors in spacing or hyphenation.

Concept / Approach:
Self evident is a classic example of a hyphenated compound adjective when used before a noun, as in a self-evident truth. In this role, self and evident combine into a single descriptive unit and are joined with a hyphen. Other compounds in the list are either incorrectly punctuated or split into two words when standard usage prefers a different form. The correct answer will be the one that matches established dictionary spelling.

Step-by-Step Solution:
Step 1: Examine option D Self-evident. This matches the familiar phrase self-evident truth, where the compound acts as a single adjective and is written with a hyphen. Step 2: Consider option A Spur of the moment-decision. Standard usage for this idea is a spur-of-the-moment decision, with hyphens joining spur, of, and the to form one compound adjective before decision. The hyphen in the given option incorrectly links moment and decision rather than the earlier words. Step 3: Look at option B Thirty one. When writing the number between twenty-one and ninety-nine in words, the standard form uses a hyphen: thirty-one. The space instead of a hyphen makes this spelling nonstandard. Step 4: Option C Quickly-running uses a hyphen between an adverb and a participle. In modern English, adverb plus participle combinations like quickly running are normally written without a hyphen. Step 5: Option E Hard ware splits hardware into two words. In standard usage, hardware is a single closed compound noun referring to physical computer components or tools.

Verification / Alternative check:
You can quickly verify these choices with the way they appear in textbooks and standard dictionaries. Phrases like self-evident truth and thirty-one students appear frequently in academic writing. In contrast, you will rarely see quickly-running with a hyphen in serious writing. Additionally, spur-of-the-moment decision is well known as a three word hyphenated compound before decision, which is different from the incorrect placement shown in option A. These checks confirm that only self-evident is correctly punctuated as a compound word in the list provided.

Why Other Options Are Wrong:
Option A misplaces the hyphen; it should connect the words in the descriptive phrase spur-of-the-moment, not moment with decision.
Option B omits the required hyphen in thirty-one, which is standard for compound numbers in that range.
Option C uses an unnecessary hyphen between an adverb and a verb form, which is typically considered incorrect in modern English grammar guides.
Option E unnecessarily splits hardware into two words, even though dictionaries treat it as a single compound noun.

Common Pitfalls:
Hyphenation rules can be confusing because many compounds gradually change form over time, moving from open to hyphenated to closed. However, exam questions usually focus on well established patterns such as hyphenated self compounds (self-evident, self-aware) and hyphenated numbers between twenty-one and ninety-nine. A useful tip is to study lists of common hyphenated adjectives and closed compound nouns so that forms like self-evident and hardware become instantly recognisable.

Final Answer:
The correctly written compound word among the options is Self-evident.