Out of the four alternatives, choose the one word which can be substituted for the phrase: "A person who is working in the same institution."

Introduction / Context:
One word substitution questions check the learner's ability to replace descriptive phrases with precise nouns. In professional and academic environments, it is useful to know the correct term for people who share a workplace or institution with us. This question focuses on the common relationship between two persons working in the same organisation.

Given Data / Assumptions:
The phrase to be replaced is: a person who is working in the same institution. The answer options include guide, collector, captain, colleague, and another general term. We assume that the institution could be a company, school, college, or other organisation, and that the relationship described is between people who are connected by their common workplace rather than by rank or friendship alone.

Concept / Approach:
The standard English word for a person who works with you in the same office, department, or institution is colleague. This term is neutral regarding rank and can refer to peers, seniors, or juniors as long as they share professional association within the same organisation or field. The other options name specific roles or unrelated positions that do not capture the general idea of working in the same institution.

Step-by-Step Solution:
Step 1: Focus on the key idea in the phrase: working in the same institution.
Step 2: Recall that colleague is used for people connected by a shared workplace or profession.
Step 3: Compare each option and reject those that indicate particular jobs like collector or captain.
Step 4: Notice that guide refers to someone who shows the way or explains things, not necessarily a co worker.
Step 5: Select colleague as the one word that exactly matches the description given.

Verification / Alternative check:
Common sentences confirm this usage: She discussed the project with her colleagues at the university, or He and his colleagues work in the same bank. In each case, colleague simply refers to another person working in the same institution or field. The term does not indicate any special function like guiding tourists or collecting revenue. This consistent pattern in everyday professional language supports the choice of colleague as the correct substitution.

Why Other Options Are Wrong:
Option A, guide, normally refers to a person who leads or directs others, such as a tour guide, and does not imply working in the same institution. Option B, collector, is a specific job title used for officials who collect revenue or items, not a general co worker term. Option C, captain, refers to a leader of a team, ship, or group, and again does not necessarily share equal institutional status. Option E, fellow, can be used informally for someone of the same kind, but it does not precisely capture the sense of shared employment within an institution. Colleague is the exact professional term that matches the definition.

Common Pitfalls:
Some candidates may be tempted by fellow because it sounds similar to expressions like fellow student or fellow worker, but in one word substitution questions the exam usually expects the most precise and widely accepted term. To improve performance, learners should create small lists of professional relationship words such as colleague, subordinate, superior, associate, and partner, along with their typical contexts.

Final Answer:
The one word substitute for a person who is working in the same institution is Colleague.