Option 1 -> Imagery appeals to senses through descriptive language, but this phrase primarily makes a comparison rather than creating sensory description.
Option 2 -> Pun involves wordplay with multiple meanings or similar-sounding words, which doesn't apply here.
Option 3 -> Metaphor directly compares two unlike things (milk and gold) without using "like" or "as" to emphasize the milk's high value.
Option 4 -> Allusion refers to an external reference (literature, history, etc.), but "liquid gold" is a direct comparison, not an indirect reference.
Hence, Option 3: Metaphor -> The phrase "liquid gold" is a metaphor that directly equates the donkey's milk with gold, emphasizing its extraordinary value (Rs 5000-7000 per litre) and precious nature without using comparative words like "like" or "as" -> correct