Nelson Mandela served as the first president of __________ from 1991-1999.  

Nelson Mandela was a pivotal figure in the anti-apartheid movement. After spending 27 years in prison, he was released and played a crucial role in negotiating the end of apartheid in South Africa. He was subsequently elected as the first democratically elected President of South Africa, serving from 1994 to 1999. While the question states 1991-1999, his presidency of South Africa is the key fact. His leadership marked a historic transition from apartheid to a multiracial democracy, earning him global recognition as a symbol of peace and reconciliation. Angola and Mauritania are incorrect as they are distinct African nations with their own political histories and leaders, unrelated to Mandela's presidency. This choice highlights his monumental legacy in establishing a non-racial democracy.