Comprehensive and Detailed In-Depth Explanation:

When using the Transit secrets engine, Vault encrypts data and returns ciphertext (e.g., vault:v1:). Upon decryption (e.g., vault write transit/decrypt/ ciphertext=), Vault returns the plaintext as a Base64-encoded string. This is because the Transit engine supports arbitrary data, including binary files (e.g., PDFs, images), and Base64 encoding ensures safe transport within JSON payloads. If the decrypted output (e.g., Zml2ZSBzdGFyIHByYWN0aWNlIGV4YW1zIGJ5IGJyeWFuIGtyYXVzZW4=) isn’t legible, it’s not an error—it’s Base64 encoded. Decoding it (e.g., using a Base64 decoder) reveals the originalplaintext (e.g., "five star practice exams by bryan krausen").

Option A (incorrect key) would cause a decryption failure, not illegible plaintext. Option B (incorrect key version) is irrelevant, as Vault automatically uses the correct version based on the ciphertext’s vault:v# prefix, and changing it manually wouldn’t produce Base64 output. Option D (database encryption) isn’t indicated in the scenario and would also cause a failure, not Base64 output. The Transit documentation explicitly states that plaintext is returned Base64-encoded, requiring the user to decode it.