Choose the statement(s) that is/are true about alpha emission.
a. alpha particles are helium nuclei.
b. alpha particles are negative electrons
c. the emission of an alpha particle results in a daughter element two units less in atomic number and four units less in mass number.
d. the emission of an alpha particle results in a daughter element one unit more in atomic number and the same mass number.

The statements a and d are correct or true .

Explanation:

Alpha  particles are doubly charged helium nucleus (He²⁺) and hence they do not have any electrons as both the electrons present in the He atom are ionized .

Alpha particle is represented by  ₂⁴He.

So the alpha particle or doubly charged Helium nucleus only have its available 2 protons along with the 2 neutrons.

So statement a is correct.

An Alpha particles not at all have any electrons so they cannot be negatively charged electrons.

So statement b is incorrect.

Since atomic number of an element= number of protons

mass number=  number of protons + number of neutrons,

Since the alpha particle has 2 protons and 4 neutrons and  hence any atom which would emit an alpha particle would reduce its atomic number by 2 and mass number by 4.

so statement c is correct.

Also statement d is incorrect.

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.