Q.planting of mustard seed
Can mustard seed be cultivated here in Nigeria? If yes, which part of the country will it thrive the most given which conditions? And why? If it can’t be cultivated in Nigeria, why can’t it? Could you please specify under which conditions one can get the best quality of mustard seed?

Yes, you should be able to grow Mustard in your region.
Here are some articles that will help you.
https://www.gardeningknowhow.com/edible/herbs/mustard/growing-mustard-seed.htm
http://www.nairaland.com/208591/healing-power-mustard-seed-it