Which of the following orders of thermal stability are correct?

Within a group, a larger cation polarises the carbonate ion less, so stability toward loss of $\mathrm{CO}{}_2$ rises down the group; options 1 and 2 follow this. Option 3 is the reverse of the true order. Option 4 is wrong because alkali carbonates such as $\mathrm{K}{}_2\mathrm{CO}{}_3$ do not decompose on ordinary heating, while $\mathrm{MgCO}{}_3$ does.