The correct answer is: unless.
The word “unless” is used to introduce a condition that must be met in order for something to happen. In this case, the speaker is saying that they will fail unless they are industrious. This means that they believe that hard work is essential to success.
The other options are not as appropriate in this context. The word “though” is used to introduce a contrast or exception. The word “until” is used to introduce a point in time at which something will happen. The word “even” is used to emphasize a point. The word “whether” is used to introduce a choice between two or more options.
Here are some examples of how the word “unless” can be used in other sentences:
- I will not go to the party unless you come with me.
- You will not be able to pass the test unless you study hard.
- I will not eat the cake unless you make it yourself.