The first African country to gain independence was:

Liberia was founded in the 1820s as a settlement for freed slaves from the United States and became a self-governing republic in 1847, when it declared independence and adopted its own constitution modelled on that of the United States. This makes Liberia the first African state to establish itself as an independent republic, decades before the wave of decolonisation that swept the continent in the twentieth century.

Ghana's independence in 1957 is frequently remembered as a landmark because it was the first country in Sub-Saharan Africa to throw off direct European colonial rule during the twentieth-century decolonisation movement, but Liberia had already existed as a sovereign, self-governed republic for over a century before that point, since it was never formally colonised by a European power in the same way. Algeria did not gain independence from France until 1962, and Egypt's path to full sovereignty stretched from nominal independence in 1922 to the final removal of British military presence in the 1950s, both well after Liberia had established itself as a republic.

The distinction the question is testing is between countries that gained independence through the mid-twentieth-century decolonisation wave and Liberia, whose republic predates that wave entirely, which is why it is credited as the first African country to gain independence.

When ranking African independence dates, remember that Liberia sits outside the usual colonial timeline altogether, since its founding and self-governance trace back to the nineteenth century rather than to the post-war decolonisation era most other African states belong to.