The plain explaination is 这是什么. But in Northern China, people always use 啥 or 嘛 to replace 什么. If you say 这是啥 or 这是嘛, Chinese people will find it funny but amazing because it's a very colloquial and native speaking way.

这是什么？

the best answer：这是什么？

Hi friend,"这是什么？”is the best answer.If you understand a few dialect like "这是啥”“这是嘛”also good when you are in north of China. Well to people from south of China, we do not feel very familiar to hear"这是啥”“这是嘛”，because it is dialect expression.We recommend mandarin like "这是什么？”so everyone knows what you are talking about!

more local one：介是嘛？or 这是嘛？ ‘