Avalanche effect in cryptography refers to:

Concept — diffusion and the avalanche effect. In a well-designed block cipher or cryptographic hash function, every output bit must depend on every input bit in a scrambled way. The avalanche effect is the diffusion property that flipping a single bit of an input — one bit of the message or one bit of the key — changes roughly half of the output bits, and changes them unpredictably. Its strict form, the Strict Avalanche Criterion (SAC), requires each output bit to flip with probability 1/2 whenever any one input bit is flipped.

Application. A correct description of this property must therefore state both halves of the pairing: the change made to the input is minimal, and the change produced in the ciphertext is large. Concretely, in DES a one-bit change in the key alters roughly 32 of the 64 ciphertext bits once all 16 rounds have run, and AES reaches comparable full diffusion within a few rounds. Drop the minimality requirement and the statement stops describing diffusion at all, because every cipher already produces different ciphertext for different input.

Cross-check — what each listed wording actually characterises:

• "Large changes in cipher text when the keyword is changed minimally" — both halves are present: a minimal key perturbation and a large ciphertext response. This is the avalanche effect, and it is the correct description.

• "Large changes in cipher text when the plain text is changed" — the large response is stated, but the minimality of the input change is not. Unqualified, this is only the ordinary dependence of ciphertext on the message that any encryption function has.

• "Large impact of keyword change to length of the cipher text" — this concerns ciphertext length. In a block cipher the ciphertext length follows from the message length, block size and padding; the key does not change it. Avalanche is about which bits change, not about how many bits there are.

• "None of the above" — does not apply, because one of the listed wordings does state the defining pairing.

Result. The avalanche effect refers to large changes in the cipher text when the key is changed minimally.

Note. The standard definition covers a minimal change in the plaintext exactly as it covers a minimal change in the key, and several references present both cases, so the wording of this item has been debated. What settles it here is the qualifier "minimally": only the key-based wording states the small-input-change requirement, while the plaintext wording leaves the size of the change unspecified. The official revised ISRO answer key for this paper scores the key-based wording.