In the following sentence, the bracketed part is grammatically incorrect. Choose the option that best improves the sentence: It called for an end to (discrimination based at gender) against those who do not conform to the gender assigned to them.

Introduction / Context:
This item tests your knowledge of prepositions used with the phrase "based on". The sentence describes a call to end discriminatory practices against people who do not conform to assigned gender roles. You must choose the correct prepositional phrase that naturally follows "discrimination" and clearly expresses the idea that the discrimination rests on gender as the basis.

Given Data / Assumptions:

• Original sentence: It called for an end to "discrimination based at gender" against those who do not conform to the gender assigned to them.
• The word "discrimination" is followed by "based", then a preposition and the noun "gender".
• Standard English requires a specific preposition with "based" in this meaning.
• The context is human rights or social justice language.

Concept / Approach:
The phrase "based on" is the correct and widely used combination when we describe the grounds for a decision or behaviour. We talk about "decisions based on evidence" or "discrimination based on race, gender, or religion". Prepositions like "at", "it", or "by" do not collocate correctly with "based" in this structure. Therefore, the only grammatically and idiomatically correct option is "discrimination based on gender". This preserves the intended meaning that gender is the reason for unfair treatment.

Step-by-Step Solution:
Step 1: Focus on the core phrase: "discrimination based ___ gender".Step 2: Recall the common pattern "based on + noun".Step 3: Check the options and identify the one that uses "on": "discrimination based on gender".Step 4: Insert that option into the original sentence and read it in full: "It called for an end to discrimination based on gender against those who do not conform to the gender assigned to them."Step 5: Confirm that the sentence now sounds natural and clearly conveys the intended meaning.

Verification / Alternative check:
Reading the sentence with the other options quickly reveals the problem. "Based at gender" does not make sense, because "at" typically indicates location or direction, not cause. "Based it gender" is ungrammatical and seems to contain a misprint. "Based by gender" is also unnatural in this context. On the other hand, "based on gender" exactly matches common usage in legal and social discussions. That confirms option B as the correct improvement.

Why Other Options Are Wrong:

• Option A: "discrimination based it gender" appears to be an incorrect combination and is grammatically wrong.
• Option C: "discrimination based by gender" does not follow any standard English pattern and sounds awkward and unclear.
• Option D: "No improvement" would keep "based at gender", which is not acceptable in standard English in this meaning.

Common Pitfalls:
Students often focus on the big nouns like "discrimination" and "gender", and overlook the small but crucial prepositions. Exam setters deliberately introduce such small mis-collocations because they are easy to miss at high speed. A useful habit is to underline or mentally highlight prepositions when you see the word "based" or similar verbs and ask yourself if the combination matches what you have seen in high quality reading material. Developing this sensitivity can greatly improve your performance in grammar sections.

Final Answer:
The correct improvement is discrimination based on gender.