The term "written constitution" refers to a constitution that has been methodically and logically recorded in a single document carrying one or more dates. Contrarily, an unwritten constitution simply indicates that they are not legally incorporated in a single book, even though they are documented. It does not imply, however, that no provisions or laws of the constitution will be discovered in written form. Written constitutions, as the word suggests, are those in which the nature of the constitutional arrangement, the laws controlling the entire system, and the rights and obligations of the citizens and the government are all included in a legal and formal book or collection of documents bound like a book.
The answer is D. The Axumite Empire adopted and followed Christianity while Songhai's was strongly influenced by the religion of Islam. Hope this helps <3