Fill in the blanks with the most appropriate option:-

We shall fail ………….. we are optimistic.

unless

The correct option to fill in the blanks is unless.

The sentence is about the condition for success, which is being optimistic. The word "unless" is the most appropriate conjunction to use in this context, as it indicates that the outcome (failure) will only happen if the condition (optimism) is not met.

The other options are not as appropriate:

• Though means "although" or "despite the fact that". It would not be accurate to use "though" in this context, as the outcome (failure) is not guaranteed to happen even if the condition (optimism) is not met.
• Since means "because" or "as". It would not be accurate to use "since" in this context, as the outcome (failure) is not caused by the condition (optimism).
• Because means "for the reason that". It would not be accurate to use "because" in this context, as the outcome (failure) is not caused by the condition (optimism).