In this one-word substitution question, choose the single English word that best expresses the phrase "mountain climbing", referring to the sport or activity of climbing high mountains.

Introduction / Context:
This question checks your awareness of a specialised term for a physical outdoor activity. Mountain climbing, especially in high and challenging ranges, is a recognised sport and adventure field with its own vocabulary. English uses a specific word that historically refers to climbing in the Alps but now more broadly denotes the sport of high-altitude mountain climbing.

Given Data / Assumptions:
The phrase given is "mountain climbing". We must choose the correct one-word term for this activity. We assume a general context of geography, adventure, or sports.

Concept / Approach:
The word "alpinism" derives from "Alps", the famous European mountain range, and originally referred to climbing in those mountains. Over time, it has come to mean the sport or art of climbing high mountains in general. The other options describe completely different concepts and are easy to rule out once you focus on the meaning of the stem and the root "Alp" in "alpinism".

Step-by-Step Solution:
Step 1: Identify that the question is asking for the name of an activity, not a medical or mathematical term. Step 2: Notice the root "Alp" in "alpinism", which suggests a link with mountains. Step 3: Recall that "alpinism" means mountain climbing as a sport. Step 4: Confirm that the other options have no connection to mountains or climbing. Step 5: Select Option B as the correct one-word substitution.

Verification / Alternative check:
You can verify by thinking of phrases like "the history of alpinism" or "modern alpinism equipment", which are clearly about mountain climbing. If you tried to replace "alpinism" there with "albinism", "alienisms", or "algorism", the meaning would become nonsensical. This confirms that "alpinism" is the only option that fits the phrase "mountain climbing".

Why Other Options Are Wrong:
Option A, "Alienisms", refers to foreign expressions or customs, not physical activities. Option C, "Albinism", is a genetic condition characterised by lack of pigment in skin, hair, and eyes. Option D, "Algorism", relates to arithmetic and calculation methods, especially using Arabic numerals. None of these terms has anything to do with climbing mountains, so they cannot substitute the phrase in the question.

Common Pitfalls:
Because several options look similar and start with "Al-", candidates may guess without carefully analysing roots and meanings. Confusing "alpinism" with "albinism" is particularly common due to visual similarity. The best strategy is to look for familiar roots: "Alps" leads to mountains, while "alb" relates to "white". Building a habit of connecting words to their origins will help prevent errors in such vocabulary questions.

Final Answer:
The correct answer is "Alpinism".