In this one-word substitution question from English vocabulary, you must choose the single word that best replaces the phrase "one who walks in sleep", that is, a person who moves about while still asleep.

Introduction / Context:
This question checks your understanding of one-word substitutions that describe specific types of people. The phrase given is "one who walks in sleep". In medical and psychological contexts, sleepwalking is a known condition. English provides a precise term for a person who does this, and recognising such terms is important for competitive exams and advanced reading.

Given Data / Assumptions:
- The phrase is "one who walks in sleep".
- The options are "Drover", "Fastidious", "Numismatist", and "Somnambulist".
- The question targets special nouns that classify people by their behaviour or interest.
- Only one option correctly refers to a person who walks while still asleep.

Concept / Approach:
The technical term for sleepwalking is "somnambulism", formed from Latin roots meaning sleep and walking. Accordingly, a person who sleepwalks is called a "Somnambulist". A "Drover" is someone who drives cattle or livestock. "Fastidious" is an adjective describing a very particular or hard-to-please person. "Numismatist" is a person who studies or collects coins, medals, or currency. Once you know the meaning of these words, the correct answer becomes clear.

Step-by-Step Solution:
Step 1: Interpret the phrase "one who walks in sleep" as a person who walks or moves about while still sleeping. Step 2: Analyse "Drover". This is a person who drives cattle or flocks, not someone who sleepwalks. Step 3: Analyse "Fastidious". This is an adjective meaning very careful, demanding, or difficult to please and is not a noun referring to a type of person by behaviour. Step 4: Analyse "Numismatist". This refers to a person who collects or studies coins and currency, unrelated to sleepwalking. Step 5: Analyse "Somnambulist". This is the noun used for a person who walks in sleep, derived from the term "somnambulism".

Verification / Alternative check:
If you look at the roots, "somnus" relates to sleep and "ambulare" relates to walking. Together they form "somnambulism" for sleepwalking and "somnambulist" for the person who does it. The other options either refer to completely different professions or are adjectives. There is no overlap between cattle driving, coin collecting, being fussy, and walking in sleep. Therefore "Somnambulist" is the only precise one-word substitution for the phrase given.

Why Other Options Are Wrong:
"Drover" is wrong because it is about handling animals, not sleep. "Fastidious" is wrong because it is an adjective describing meticulous or overly picky behaviour, not a specialised role or condition. "Numismatist" is wrong because it describes a coin collector or researcher. None of these connect with the act of walking while asleep, which is a specific sleep disorder.

Common Pitfalls:
Learners sometimes pick options that simply sound technical or unusual, thinking they must be correct. However, in one-word substitution questions, each option usually has a very specific meaning. Memorising a core list of such words with clear definitions is very helpful. Always match the phrase carefully to the exact meaning of the option instead of guessing based on sound alone.

Final Answer:
The correct one-word substitution for "one who walks in sleep" is Somnambulist.