Answer:
conjunction
Step-by-step explanation:
The sentence begins with a conjunction, but.
But as a conjunction is used when introducing a statement that is added. The added statement may be something different from what you have said before.
It is used to connect ideas that actually contrast.
In the sentence given, we discover that "but" was used to introduce the sentence that follows it. It then means that something has been stated before "but" was added.