C -> correct

The correct answer is C. myArray.flat(Infinity);

The Array.prototype.flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth. The Infinity value for the depth argument specifies that the array should be flattened all the way down, regardless of how many levels of nesting it has.

Option A is incorrect because the .join() and .split() methods do not flatten an array. They simply create a new string by concatenating all the elements of the array with a separator and then split the string into an array of substrings based on the separator.

Option B is incorrect because the spread operator (...) can only be used to expand an array into its individual elements, not to flatten it.

Option D is incorrect because the .reduce() method does not have a built-in way to flatten an array. It simply applies a callback function to each element in the array to reduce it to a single value.

The correct answer should be C: myArray.flat(Infinity);