In computational linguistics, what does 'Backoff' refer to in the context of N-gram models?

1. ক
   Using lower-order models when higher-order data is sparse
   
2. খ
   Removing infrequent words from the vocabulary
   
3. গ
   Reversing the order of words in a sentence
   
4. ঘ
   Ignoring the grammatical structure of the input