Answer:
Exclusively.
Step-by-step explanation:
Let's take a look at these two words in the sentence.
Too many people concentrate almost (exclusive, exclusively) on losing weight rather than on building health.
Here it is with the word exclusive:
Too many people concentrate almost exclusive on losing weight rather than on building health.
Now if we look at that sentence, it doesn't make any grammatical sense.
Now, here it is with the word exclusively:
Too many people concentrate almost exclusively on losing weight rather than on building health.
This seems to make sense to a reader and the person writing it, as well as making grammatical sense.
So, I believe the correct answer is exclusively.