Final answer:
The expression (int) 4.2/5 results in 0.8 after truncating the decimal part to an integer which gives 4, and then dividing by 5.
Step-by-step explanation:
The result of the expression (int) 4.2/5 is determined by first considering the cast to an integer type which truncates the decimal part of the number 4.2 (making it simply 4) before the division is performed. Therefore, the calculation becomes 4 / 5, which is a mathematical evaluation yielding a result of 0.8. So, the correct answer is option b) 0.8.