An element's atomic number is the a. number of protons in an atom's nucleus. b. mass of the atom. c. weight of the atom. d. sum of the protons and neutrons in an atom

The atomic number defines the number of protons within the nucleus of the atom. Each atom has a unique number of protons in its nucleus.
Examples:
Hydrogen has an atomic number = 1, this means that the number of protons within the nucleus of the hydrogen is 1.
Carbon has an atomic number = 6, this means that the number of protons within the nucleus of the carbon is 6.

Based on this, the correct choice would be:
A. number of protons in an atom's nucleus.

If the ice absorbed 350,000 joules in 5 minutes, then it absorbed energy
at the rate of

   (350,000 joules) / (5 x 60 seconds)  =  1,166-2/3 watts .

Surely the ice cube didn't absorb every joule delivered to the cooking chamber,
so the microwave oven's cooking power had to be significantly more than that.