B, arrow functions are my go to function syntax, it is far easier to read and follow. Plus template literals are awesome for writing long codes and creating DOM elements. For one into reactJs, it gives that jsx feeling
In this short example it doesn't matter that much. But for a more complex concatination B is the correct way to go. No worries about whitespace, functions and a is for additions.
You are on the right track. Not only B is using template literals but also it is taking advantage of arrow functions which are both great capabilities offered by es6.