Among the solutions Josh is testing, the one that conducts electricity is the solution containing salt (sodium chloride, NaCl). This is because when salt is dissolved in water, it dissociates into its constituent ions (Na+ and Cl-), which are charged particles that can move freely in the solution. The movement of these ions allows the solution to conduct electricity.
On the other hand, sugar (sucrose) does not dissociate into ions when dissolved in water; it dissolves as whole molecules. Therefore, a sugar solution does not conduct electricity. Similarly, alcohol (such as ethanol) also does not dissociate into ions in solution and does not conduct electricity.
In summary, only the salt solution conducts electricity due to the presence of mobile ions.