In the following question, out of the four alternatives, choose the one which can be substituted as a single word for the description: "The belief that everyone is equal and should have the same rights and opportunities."

Introduction / Context:
This one-word substitute question tests your knowledge of political and social vocabulary. The description is "The belief that everyone is equal and should have the same rights and opportunities." This is a core idea in discussions about democracy and social justice, and English provides a specific word to label such a belief or the person who holds it.

Given Data / Assumptions:

• The phrase focuses on equality and equal rights.
• It refers to a belief system or attitude about society.
• Options include Altruistic, Egoistic, Egalitarian, Octogenarian, and Aristocrat.
• We assume typical exam level knowledge of ideological adjectives.

Concept / Approach:
The correct word here is "egalitarian". An egalitarian believes in or is based on the principle that all people are equal and deserve equal rights and opportunities. The root "egal" is related to "equal". "Altruistic" refers to selfless concern for others, "egoistic" to self interest, "octogenarian" to a person in his or her eighties, and "aristocrat" to a member of the privileged upper class. None of those words capture the specific principle of equality for all.

Step-by-Step Solution:
Step 1: Identify the key words in the description: everyone is equal, same rights, same opportunities.Step 2: Recall political vocabulary where equality is central, such as egalitarianism.Step 3: Recognise that a person or system that accepts this belief is called "egalitarian".Step 4: Scan the options to find that exact term.Step 5: Cross check that other options express very different ideas and therefore can be rejected.

Verification / Alternative check:
Consider a simple example sentence: "The new policy is based on egalitarian principles, giving all citizens equal access to education." Here, "egalitarian" clearly refers to equal rights and opportunities. If you substitute "altruistic" or "aristocratic", the sentence changes its meaning completely. Altruistic policies emphasise helping others selflessly, which is not the same as structural equality, and aristocratic policies would favour the upper class, exactly the opposite of equality.

Why Other Options Are Wrong:

• Altruistic: Refers to unselfish concern for the welfare of others. It is about kindness and generosity, not specifically about equal rights for all.
• Egoistic: Relates to self interest and selfishness, the opposite of caring about equal rights for everyone.
• Octogenarian: Describes a person who is between eighty and eighty nine years old. It has nothing to do with beliefs about equality.
• Aristocrat: A member of a hereditary upper class or nobility. Aristocracy is usually associated with inequality of rights and privilege, not equality.

Common Pitfalls:
Some test takers confuse "altruistic" with egalitarian because both seem to express positive attitudes toward others. However, altruism is about personal kindness and self sacrifice, while egalitarianism is about a political or social principle of equal treatment. In one-word substitute questions, read the description carefully and look for precise matches rather than general positive words.

Final Answer:
The correct one-word substitute for the belief that everyone is equal and should have the same rights and opportunities is Egalitarian.