Let the total number of questions in the test be x. As the candidate attempted 12 questions, he left out (x - 12) questions.

As he secured full marks in all 12 questions attempted, his total score from these questions is 12 (assuming each question carries 1 mark).

Let the marks for each question be m. Then, the total marks in the test is x m.

As the candidate obtained 60% in the test, his total score is 0.6 times the total marks in the test. Therefore, we can write:

12m + (x - 12) * 0 = 0.6 * xm

Simplifying this equation, we get:

12m = 0.6xm

Dividing both sides by 0.6m, we get:

20 = x

Therefore, the total number of questions in the test is 20. The correct answer is option (D) 20.