The question about a prerequisite for the development of a thunderstorm, it says the answer is it needs to be conditionally stable atmosphere, but from what I read in the book its supposed to be conditionally unstable. the answer given is: The conditions necessary for the formation of a thunderstorm are:

1. Conditionally stable atmosphere.
2. High humidity.
3. A trigger mechanism to begin the initial lifting of the air.

Conditional stability exists when dry air is stable but saturated air is unstable. That is, rising air that doesn't form cloud will eventually stop rising and cause no real problems for a pilot. However, once cloud begins to form, the cloud will become unstable and rapidly grow to form a large cumulonimbus cloud.
The correct answer is: a conditionally stable atmosphere

So just wondering which one is the correct answer?

Condition stability is as you described. The atmosphere is stable if the rising air parcel is dry but unstable if the rising air parcel is saturated. In short, "conditionally stable" and "conditionally unstable" mean the same thing.

The atmosphere is "conditional stability" when the ELR lies between the dry adiabat and the moist adiabat - see diagram below.

To get a thunderstorm you need three ingredients.
1) you need atmospheric instability - absolute instability or conditional instability
2) you need fuel (water vapour)
3) you need a spark (a lifting mechanism).

That means answers 1, 2 and 3 are all technically correct - something to think about.

Thanks! I think the wording was just confusing me.

Dont worry KateM, the "UN" (in front of stable) has been confusing the heck out of me as well... I started ignoring it and just looking for the "Conditionally" bit

I passed the met exam, thank god I asked because the question came up!

I also got distracted by the term 'conditionally stable'. It is not used anywhere in Bob's CPL Meteorology book, except in one of the practice tests where it is a distractor (wrong answer). But I found several sources online which use the term 'conditionally stable' as equivalent to 'conditionally unstable'. So, it might come up in the CASA exam. Just read the 'conditionally'..