Altimetry
I tend to agree that 'd' is the correct answer to the first question. Like most, I am a bit confused by the term 'isobaric surfaces' (Must ask the met lecturer tomorrow!) but, assuming that the term means the distance between 2 pressure surfaces, the altimeter indicates height/altitude etc of (usually) one above the other, calibrated to ISA conditions. In other words, once you have set whatever datum pressure is in use, the altimeter makes all its calculations based on ISA pressure changes, regardless of the actual atmospheric conditions.