The question relates with margins of error.
The question gives the answer for the QNH margin of error : +/-5hPa is +/-150ft.
The altimeter is said to be perfect. However, is the static pressure source perfect ? I will assume so. In real life, there will always be a small error due to propwash on the ground and relative wind in the air.
I don't see any other source of error (instrument misread maybe?).
So theoretically the answer would be b), or D), there is always a margin of error but hopefully much lower than 150ft in this case.
But, I would add something. QNH represents the air surface pressure. It's something that evolves slowly over large distances. So it's rather continuous. 5hPa of error seems very big. Plus, QNH can be measured accurately at multiple locations to help the forecast. In practise, I rarely noticed more than 1-2 hPa of error when applying the right QNH on the ground when compared to the charted airfield altitude.
For the second question I will ignore this remark and assume my first answer was b)
B for the second question would be tempting, but if there is a measurement error on the ground, I think this error will only ever increase with altitude, because if 1hPa is 30ft close to the ground, it's more (much more) up in the air. The effect won't be very noticeable at 500ft (1hPa = 30.5ft) so the answer is close to b) but I specified some aspects according to d)