Step 1: Understanding the Concept:
This question tests grammatical correctness and the proper use of concession conjunctions in complex sentences.
Concession clauses are used to join two contrasting ideas where one action makes the other action seem surprising or unexpected.
Step 2: Detailed Explanation:
The given sentence has two contrasting parts: "he worked hard" (a positive effort) and "he failed in the examination" (a negative outcome).
To connect these contrasting statements grammatically, we must use a concession conjunction.
Let us analyze the options:
- Even: The word "Even" cannot act as a subordinating conjunction on its own to introduce a clause of concession.
It would require the word "though" to form "Even though" to be grammatically correct in this context.
- Since: This is a causal conjunction used to indicate reason or time (similar to "because").
Using "Since" would suggest that working hard caused him to fail, which is logically incorrect.
- For: This is a coordinating conjunction used to state a reason.
It is equivalent to "because" and does not express the required contrast or concession.
- Although: This is a subordinating conjunction used to introduce a concession clause.
It is grammatically complete on its own and fits perfectly: "Although he worked hard, he failed in the examination."
Thus, "Although" is the only grammatically and semantically correct substitution.
Step 3: Final Answer:
The correct substitution is "Although."