Create document from nested array content

jq 'map(if (.response|type == "array") then .response = .response[] else . end)' file

This tests the type of the response entry in each element of the top-level array. If it's an array, the entry is "exploded" or duplicated once for each array element; otherwise, it's left as-is.

The result, given the data in the question (with a problematic comma removed):

[
  {
    "command": "1",
    "response": "a",
    "guild_id": "guild"
  },
  {
    "command": "1",
    "response": "b",
    "guild_id": "guild"
  },
  {
    "command": "1",
    "response": "c",
    "guild_id": "guild"
  },
  {
    "command": "1",
    "response": "d",
    "guild_id": "guild"
  }
]

The same thing, but a bit more cryptic:

jq 'map(.response = .response[]? // .)' file

This duplicates (in the same sense as above) the entry from the top-level array if its response entry is an array; otherwise, it's left as-is.