Step 1: Understanding Apohavāda.
Apohavāda, also known as Apohasiddhānta, is a prominent Buddhist theory of meaning (semantics) notably expounded by Dignāga and Dharmakīrti. This theory posits that words denote an object not through direct naming but by excluding all other possibilities (apoha means exclusion).
Step 2: Meaning of Atadvayāvartaka.
The core concept of Apohavāda is that a word signifies "exclusion of the other" (anyāpoha). This is termed Atadvayāvartaka, signifying "exclusion of the double negation." In essence, saying "cow" implies "not non-cow."
Step 3: Elimination of incorrect options.
- (1) Asaṅkhyeya: Refers to countless or innumerable; irrelevant to this context.
- (2) Avarṇanīya: Means indescribable; unrelated to the Apoha theory.
- (4) Asadvayāvartaka: An incorrect term, not employed within the Apoha theory.
Step 4: Conclusion.
Therefore, Apohasiddhānta is identified as Atadvayāvartaka.
Final Answer: \[\boxed{\text{The correct answer is (3) Atadvayāvartaka.}}\]