how do i find out which item in a dictionary is the longest

For a Python dictionary, you usually want the key whose value is longest. The simplest way is:

python

longest_key = max(d, key=lambda k: len(d[k]))

If you want the key and the value together:

python

key = max(d, key=lambda k: len(d[k]))
value = d[key]

Examples

python

d = {
    "a": [1, 2],
    "b": [1, 2, 3, 4],
    "c": [9]
}

longest_key = max(d, key=lambda k: len(d[k]))
print(longest_key)   # b
print(d[longest_key]) # [1, 2, 3, 4]

If you mean the longest key string itself, use:

python

longest_key = max(d, key=len)

That works because max can compare the keys directly by their string length.