He's welcome to come along, provided that he behaves himself. Here the underlined part is a/an -

• সঠিক উত্তর হলো - Compound conjunction.
- যে সকল phrase, conjunction হিসাবে ব্যবহৃত হয় তাকে Compound conjunction বলে।
- Examples: so that, provided that, as well as, as soon as, as long as, such that, in order that etc.
- তাছাড়া -
- A compound conjunction may have two or three parts and they always go together. They are different from correlatives which are conjunctions used -only in pairs. Examples of correlatives are: either…or, neither…nor, not only…but also.

সূত্র: Advanced Learner's Grammar and Composition by Chowdhury and Hossain (HSC English).