Choose from the options given below, the word that can be used to fill in the blank in the given sentence.

______he worked so hard, he couldn't pass the examination.

Unless

The correct answer is Option (2) → Although

Although (or 'though' or 'even though') is a conjunction used to introduce a subordinate clause which expresses a concession or a contrast. It indicates that despite the fact that he worked hard, the unexpected outcome was that he couldn't pass.

Let's look at why the other options are incorrect:

• Unless: Means "except if." "Unless he worked so hard, he couldn't pass" doesn't make logical sense in this context.

• Until: Means "up to the time that." "Until he worked so hard, he couldn't pass" implies that working hard was a condition for not passing, which is illogical.

• Provided: Means "on the condition that." "Provided he worked so hard, he couldn't pass" suggests that working hard resulted in failure, which is contrary to the usual expectation.