In the following question, out of the four alternatives, select the alternative which is the best one word substitute for the phrase: An old unmarried woman.

Introduction / Context:
Here you are asked to choose a single word that can replace the descriptive phrase an old unmarried woman. One word substitution questions test your ability to recall precise vocabulary for common definitions. Although some of these terms may be old fashioned or sensitive in modern usage, they are still tested in many competitive exams.

Given Data / Assumptions:
- Phrase: An old unmarried woman. - Options: Masochist, Septuagenarian, Sniper, Spinster. - We must select the word that refers to an older woman who has never married.

Concept / Approach:
Spinster is a traditional term for a woman who is unmarried, especially one who is past the usual age for marriage according to older social expectations. Although the word can sound negative or outdated in everyday conversation today, it remains a standard answer in exam oriented English. The other options describe completely different ideas, such as enjoying pain, being in a certain age group or being a shooter, and therefore cannot fit the given definition.

Step-by-Step Solution:
Step 1: Understand the phrase an old unmarried woman as a woman who has remained unmarried beyond the typical marriage age. Step 2: Examine Masochist. A masochist is someone who derives pleasure from suffering pain or humiliation. This is a psychological term, not related to marital status. Step 3: Examine Septuagenarian. A septuagenarian is a person who is between seventy and seventy nine years of age. This term indicates age but not marital status and can apply to both men and women. Step 4: Examine Sniper. A sniper is a person who shoots at targets from a hidden position, usually in military or police contexts. This has no link to being unmarried. Step 5: Examine Spinster. Spinster is used to refer to an unmarried woman, typically older and beyond the usual marrying age.

Verification / Alternative check:
Use the word in context. Older novels and legal documents may refer to a woman as a spinster to indicate that she is unmarried. If we replace the phrase an old unmarried woman with spinster in a sentence like The property was left to an old unmarried woman, it becomes The property was left to a spinster, which conveys the same essential information. The other options cannot be used in that way without changing the meaning entirely.

Why Other Options Are Wrong:
Masochist is wrong because it describes a person who enjoys pain or humiliation and does not refer to marriage. Septuagenarian is wrong because it only tells us that the person is in their seventies, not whether they are married or not. Sniper is wrong because it denotes a skilled shooter and has nothing to do with age or marital status.

Common Pitfalls:
Some learners may hesitate to choose spinster because they know it can be considered old fashioned or even impolite in normal conversation. However, exam questions are often based on traditional dictionary definitions rather than current social preferences. It is important to separate real life politeness from exam oriented vocabulary. Knowing words like bachelor and spinster and understanding where they are still used is useful for scoring well in one word substitution questions.

Final Answer:
The correct one word substitute for “An old unmarried woman” is Spinster.