Step 1: Understanding the Concept:
Check if every letter in the options exists in the keyword "PRESIDENTIAL" and in the correct quantity.
Step 2: Detailed Explanation:
- ARDENT: A, R, D, E, N, T (All present).
- SLEEP: S, L, E, E, P (All present).
- DENTAL: D, E, N, T, A, L (All present).
- PRESIDE: P, R, E, S, I, D, E (All present).
- DIGITAL: D, I, G, I, T, A, L.
Step 3: Calculation:
The word "DIGITAL" contains the letter 'G'. There is no 'G' in "PRESIDENTIAL."
Step 4: Final Answer:
DIGITAL cannot be formed. Thus, the correct option is (a).