Which of the following is a valid 2D array initialization in C?

Specifying both dimensions arr[rows][columns] and using nested braces {} is the clearest way to initialize the structure:

int arr[2][2] = {{1, 2}, {3, 4}};

Omitting the First Dimension:** C allows you to leave the *row* dimension empty if you provide an initializer list, because it can deduce the number of rows based on the data. However, the *column* size remains strictly mandatory:
```c
int arr[][2] = {{1, 2}, {3, 4}}; // Also valid!


You can also initialize a 2D array using a single flat list of elements inside curly braces, as long as the column dimension is specified:
int arr[2][2] = {1, 2, 3, 4}; // Also valid (implicitly mapped row-by-row)