In Minesweeper, squares with numbers don't have mines, but the number indicates how many of the eight squares surrounding that square contain a mine. That means the hallways are all mine-free because they each have a "0" in their center.
For each room, try to deduce as much as possible which squares have mines and which groups of squares must have a certain number of mines in them. Then, find a path through the room that is least likely to contain a mine. Don't assume that a mine-free path exists!
Literally, "what are the chances"? For each room, calculate the probability (as a fraction) that the safest possible path is mine free. Assume each valid configuration of mines is equally likely.
Each probability is of the form 1/n, where n is a number from 1 to 26. The final answer is each denominator converted to a letter using the A1Z26 cipher read in order of the rooms you traversed.
One of the rooms has a path that you can guarantee is safe with probability 1/1, meaning it corresponds to the letter "A." Two other rooms have a path that is mine-free with probability 1/2. Remember that the placement of mines may depend on the placement of other mines, so for some rooms you may have to list out each possible mine configuration to see how many of them make your path to be safe.
The trickiest room is the last one. The safest path is up past the two 1's, left through the 3, up to the 1, and then right to the 4. But due to the mine placement around the 1 depending on the mine placement around the 3, this path only has a 1/5 chance of being safe, making the last letter in the answer an "E."