taru-adjective, adverb taking the 'to' particle
Literary たる adjective describing a state where something is plainly apparent. Often used in the adverbial form 瞭然と.
彼の嘘は瞭然としていた。
His lie was obvious.
証拠は瞭然としている。
The evidence is clear.
明らか is the common, everyday word for 'obvious' or 'clear'. 瞭然 is a stiff, literary expression used mainly in formal writing.
From Chinese 瞭然/了然. The exact historical derivation is uncertain; the spelling is conventionally associated with clarity and obviousness.