I am sure you know that the answer is A - Exhaust. But getting there took some head scratching. But I believe I got it, but probably would not in an exam. I will try to convey what I did.
The firing order is for an eight cylinder so I assume that each turn of the crank has four cylinder firing, and the others on the exhaust or compression strokes. So all cylinders will have fired every two rev. In the first turn of the crank 1526 are firing, then the next turn of the crank 8473 are firing each at 90 degree intervals. If you had a six cylinder, you would have three firing each rev at 120 degrees apart. This is to make sure the engine balanced.
Start at TDC Cylinder 1 - 1 starts power stroke, 8 starts intake; 90 degree - 5 is on power stroke, 4 intake; 180 degrees - 2 is on exhaust, 7 is in compression; 270 degrees - 6 is in exhaust, 3 on compression.
Crank is finished one rev, so now 8 starts power stroke, 1 starts intake; 90 degree - 4 is on power stroke, 5 intake; 180 degrees - 7 is on exhaust, 2 is in compression; 270 degrees - 3 is in exhaust, 6 on compression
Its more of a mental puzzle, which I hate, personally, but you can work it through logically. If you draw it out it makes a bit more sense.
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