This is not an issue, this is a question ;-)

Your code is incorrect and you have to debug it
I see several mistakes :

• you are incrementing the word count at each space that you encounter, but words can be separated by more than one space : "a b"
• you are allocating a fixed size buffer (malloc(256)) without being sure it will be enough in the worst case scenario
• there is a mismatch between the type and the size you allocate for it there:

  char (*words)[len] = malloc (sizeof(char[word_count][len]) );
  

  words is a pointer to a char array of length len
  you allocate the size of an array of length word count of array of length len of char
  also note that you forgot to add +1 for the nul terminator

Your approach is too complicated. You dont need to allocate a 2D array to store every word. You can solve this in only 1 pass of the input string, keeping track of the current longest word as you go

same here.

"
The expression (as strings) (res) == ("taxi") is false: actual=taxi expected=taxi.
"

yes

Don't give free hints and let people solve the kata by themselves, that's part of the learning process, just like it has been for you. I marked your message as a spoiler.