Lets take this further:
Why is it that the BOP always balances under a floating exchange and not a fixed exchange?
Under a fixed exchange, you could have a CAD larger than the C&FAS
In which case debits exceeded credits.
Therefore, in the market forces, debits + K outflows > credits + K inflows.
Thus, the BOP would not have balanced.
UNDER A FLOATING EXCHANGE, SUPPLY = DEMAND.
Therefore, since supply exceeds demand, there would be downward pressure on the exchange rate. The exchange rate depreciates.
In the process, credits + K inflows INCREASE in value, (as the overseas currencies can purchase more Aus$)
and debits and K outflows DECREASE in value. (as the Aus$ can purchase less overseas $)
Thus, the Increase in credits and K inflows will reduce the size of the CAD, and increase the C&FAS. Thus, the C&FAS will be equal to the CAD.
I've included all this to show WHY it is true that the excess of debits will increase the dollar. The simple reason will do in an essay.
But the investor confidence does raise the fact that it is the K inflows and outflows that influence most of the dollar. (You can apply that situation to the one above. Just switch the two around from the CAD to the C&FAS)
However, the wording of the syllabus wants it from the CAD's perspective. ie. why does the CAD cause a depreciating dollar.
Though investment and speculation is correct as the major influences on the dollar, that would really be the correct answer for why a C&FAS causes a depreciating dollar.
I know its pedantic, but I hope that helps.