In the sentence "Always speak a truth," the word 'a' is incorrect.
The word 'truth' is an uncountable noun when referring to the concept of truth or fact in general. Uncountable nouns, such as "water," "information," and "truth," do not require an article ("a" or "an") before them.
The correct sentence should be:
“Always speak the truth.”
Here, 'the' is used because it refers to a specific truth, typically understood as the universal or moral truth.
Therefore, the error in the sentence is the word 'a', and it should be replaced with 'the'.
Final Answer: a