Significant digits are used to limit a number to express its true level of accuracy. The number 240 has two significant digits, the two and the four. The zero is not considered significant in this instance because trailing zeros are ignored unless a decimal point is present.
To get 240 down to one significant digit, we need to round it off. Since the second digit is a 4, we truncate, and our answer is 200. (Important note - If the second digit was a 5 or greater, we would round up and our answer would be 300.)
An easy way to remember how to count sig figs is the Atlantic/Pacific rule. Picture a map of the US, with an ocean on each side. Then look at your number. If the decimal point is Present, only ignore zeros on the Pacific (left, or leading) side of the number. If the decimal point is Absent, only ignore zeros on the Atlantic (right, or trailing) side of the number.
To make 240 to be one significant figure, the answer would be 200.
As stated in the previous post, to get the number 240 to one significant digit, you need to round it off. Hence, the answer would be 200.