What is it called when an unwanted material that contains radioactive atoms is present on another material?

Radioactive contamination

Explanation:

radioactive contamination is when radioactive atoms are present on something

The sample with the lowest specific heat capacity will have the greatest temperature increase. in this case, it would be platinum.

Ohm's law tells you that i = v/r where v is equivalent voltage and r is equivalent resistance of the circuit
since all circuits have the same voltage, your question is really asking which circuit has the lowest total resistance, since the lower the resistance, the higher the current
circuit a has a resistance of 5 ohms; circuit c is in series so has a resistance of 10 ohms; circuit b is in parallel and therefore has a resistance of 2.5 ohms (review your notes on adding resistances in parallel and series); therefore b has the highest current