English is Nigeria's official language. It is not what most Nigerians speak. Those are two very different claims, and quizzes usually mean the second one.
Nigeria's official language is English. Government business, the courts, the national press and most secondary education run in it. On paper it is about as officially English-speaking as a country can be.
And yet, in a list of countries where English is the main language, Nigeria doesn't belong — because "official" and "most widely spoken" are different measures, and they disagree far more often than people expect.
Nigeria has somewhere around 500 living languages. Hausa, Yoruba and Igbo each have tens of millions of first-language speakers, and each dominates a different part of the country. Nigerian Pidgin, an English-based creole, is spoken very widely as a common tongue — but it is its own language rather than English, mutually unintelligible with standard English in full flow.
English is the language that lets a Hausa speaker from Kano and a Yoruba speaker from Ibadan do business. That is a real and important role. It is not the same as being the language most people speak at home.
In most of these cases the answer is colonial history plus arithmetic. When a country contains hundreds of language communities and no single one holds a majority, adopting any of them as the national language is a political decision with losers. Adopting the coloniser's language is, awkwardly, the neutral option — it disadvantages everyone equally.
That is why English is official in so many countries where almost nobody speaks it first, and why the same pattern repeats with French across much of West Africa.
India has English as an associate official language, a vast English-speaking elite, and a publishing industry to match — and Hindi alone has several times more first-language speakers than the entire population of most countries on any English list.
Pakistan uses English throughout government and law; Urdu and Punjabi are what people speak. Ghana, Kenya, Uganda, Zambia and Zimbabwe all have English as an official language and local languages as the ones heard in the street — Twi, Swahili, Luganda, Bemba, Shona.
Singapore is a genuinely difficult case. English is an official language, the language of instruction, and increasingly the language spoken at home — but it shares official status with Mandarin, Malay and Tamil, and the everyday register is often Singlish. South Africa has eleven official languages, and English is not the largest of them; Zulu and Xhosa both have more first-language speakers.
The Philippines has English as co-official with Filipino, and English fluency is high — but Tagalog, Cebuano and a dozen other languages are what people grow up speaking. Botswana uses English officially while Setswana is near-universal. Malta has English alongside Maltese, which is the everyday language of the islands. Cameroon is officially bilingual in English and French, with hundreds of local languages underneath.
Ask "what language do most people here actually speak?" rather than "what does the constitution say?". The first question is the one a geography quiz means, and it gives a shorter and much less obvious list than people expect.
It is also the more interesting question, because the gap between the two is a map of empire — the countries where the official language and the spoken language pull apart are almost exactly the countries that were governed by someone else.