The sentence "Always speak a truth" is incorrect because the word "truth" is an uncountable noun and does not take an indefinite article "a". The correct sentence would be "Always speak the truth." \n\n\\begin{itemize}\n \\item always: This is correct as it indicates frequency and doesn\'t affect the grammar of the sentence.\n \\item speak: This is correct; it is the appropriate verb in the sentence.\n \\item a: This is the error. The article "a" is incorrect before "truth," which is uncountable.\n \\item truth: This is correct as a noun, but it doesn\'t take the article "a" in this context.\n\\end{itemize}\n\n\n\n \\boxed{\\text{a}}