Let 'M' represent the marks allotted for each question. Let 'N' be the total number of questions in the test.

The candidate attempted 8 questions and secured 50% marks in each.
The total marks obtained by the candidate from these 8 questions can be calculated as: $8 \times (0.50 \times M) = 4M$ .
The problem states that the candidate obtained a total of 40% in the test. The total marks for the entire test would be $N \times M$ .
Therefore, the marks obtained ( $4 M$ ) constitute 40% of the total test marks ( $N \times M$ ).
This relationship can be expressed as: $\textbf{4M = 0.40 \times (N \times M)}$ .
Since 'M' represents marks per question and cannot be zero, we can divide both sides of the equation by 'M': $\textbf{4 = 0.40 \times N}$ .
Solving for N: $N = \frac{4}{0.40} = 10$ .
Hence, there were 10 questions in the test.

If the total number of questions was 8 (Option A), it would imply the candidate attempted all questions. In this scenario, scoring 50% in each question would result in a total score of 50% for the test, which contradicts the given condition of 40%.
If the total number of questions was 15 (Option C), the total marks for the test would be $15 M$ . The percentage obtained by the candidate would be $\frac{4 M}{15 M} \times 100 = \frac{4}{15} \times 100 \approx 26.67%$ , which is not 40%.
If the total number of questions was 16 (Option D), the total marks for the test would be $16 M$ . The percentage obtained by the candidate would be $\frac{4 M}{16 M} \times 100 = \frac{1}{4} \times 100 = 25%$ , which is not 40%.