The correct option is A Hardly any effort
Let’s examine the sentence: “The old man felled some trees in the garden with hardly no effort at all.” The adverb “hardly” itself has a negative meaning as it implies “scarcely” or “no effect at all.” “Hardly” is used with a positive word and not a negative word. So, to correct the sentence the negative word ‘no’ would be replaced by a positive word ‘any.’ The correct sentence would be, “The old man felled some trees in the garden with hardly any effort at all.” Hence, Option B is the correct answer.