Conjunction in the Sentence:
The conjunction in the given sentence is "or".

Explanation:
A conjunction is a word that connects words, phrases, or clauses within a sentence. It is used to show the relationship between the connected elements. In this sentence, the conjunction "or" is connecting two clauses: "You must study hard" and "you will fail the exam".

The conjunction "or" is a coordinating conjunction, which means it connects two alternatives or choices. It indicates that one of the two clauses can be true, but not both at the same time. In this sentence, the conjunction "or" is used to present two options to the reader:

- "You must study hard"
- "You will fail the exam"

The use of "or" implies that if the person being addressed studies hard, they will not fail the exam. Conversely, if they do not study hard, they will fail the exam. It presents a conditional relationship between the two clauses.

Example:
This can be further understood by the following example:
- "You must eat your vegetables or you won't get dessert."

In this example, the conjunction "or" connects two clauses:
1. "You must eat your vegetables"
2. "You won't get dessert"

The use of "or" indicates that if the person eats their vegetables, they will get dessert. However, if they do not eat their vegetables, they will not get dessert. It presents a condition and an alternative outcome.

Therefore, the conjunction "or" in the given sentence connects the two options or choices and presents a conditional relationship between them.