‘Not only Anis but also his brothers performed their duty properly.’ This is a –

Compound Sentence is a sentence that connects two independent clauses, typically with a coordinating conjunction like ‘and, or, but’.
আলোচ্য বাক্যে ‘not only’---‘but also’ coordinating conjunction দ্বারা দুইটি independent clause যুক্ত হয়েছে।
অতএব, এটি একটি Compound sentence।