He always practised ______ ahimsa.

Concept

Articles (a/an/the) come before nouns, and the choice depends on the kind of noun. An abstract / uncountable noun (a quality, idea or principle, such as honesty, courage or non-violence) used in a general, unqualified sense takes no article (the zero article). An article appears only if such a noun is made specific by a following phrase or clause.

Application

In "He always practised ______ ahimsa", the word ahimsa means the principle of non-violence — an abstract, uncountable noun used here in a general way (he followed non-violence as a whole, not one particular instance of it). Because the noun is general and not qualified by any defining phrase, the slot is filled by the zero article: He always practised ahimsa. Hence no article is needed.

Why the article forms do not fit

• "a" and "an" are the indefinite articles, used only with a singular countable noun (a book, an idea). "ahimsa" is uncountable and cannot be split into single units, so neither indefinite form can attach to it — the choice between "a" and "an" never even arises.

• "the" is the definite article and would be needed only if the noun were made specific, e.g. "the ahimsa preached by Gandhi". With the bare, general "ahimsa" there is nothing to make it definite.

Cross-check

Compare the parallel sentences "She values honesty" and "He showed courage": each abstract noun stands alone with no article. "He always practised ahimsa" follows exactly the same pattern, confirming the zero-article reading.