Answer: but.
Explanation: In the sentence above, the correct word that complete the underlined space is the conjunction "but".
The word but is used as a conjunction to introduce a phrase or statement, usually something that is different from what you said before.
In the referred sentence we can notice that the business expectations of their company didn't go so good as they expected, BUT, on the other hand, they are also happy due to the increasing of their costumers.
So, we can see twoo different ideas in the same sentence that are united by the conjunction but.