Step 1: Define the target word Lucid.
"Lucid" comes from the Latin "lux" meaning "light"; it means clear, easy to understand, and logically expressed - a lucid explanation shines light on the subject and leaves no room for confusion.
Step 2: Identify synonyms among the options.
"Clear" and "Transparent" are both synonyms of Lucid; "Bright" is related to light and similarly positive in tone; none of these can be the antonym.
Step 3: Identify the antonym.
"Obscure" means not clearly expressed, difficult to understand, or hidden from plain view - the direct opposite of Lucid.
\[ \boxed{\text{Obscure}} \]