The mass of a sample weighed on an electronic balance that is sensitive to 0.3 mg is 1.2300 g. the number of significant figures in this measurement is

The sensitivity of electronic balance is 0.3 mg

Converting 0.3 mg to g:

g

So, the mass of a substance weighed on the balance can be measured accurately up to four decimal places.

Given the mass of sample is 1.2300 g. The number of significant figures in the measurement will be 5 because the balance can measure the mass of a sample accurately up to four decimal points, all the digits in the measurement will be significant.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.