Fleas are difficult to squish because they have
 I. Sclerites
 II. Tough spines
 III. Resilin in their joints
 A. I only
B. I & II only
C. II & III only
D. I, II and III

Hint:The word stuffed ought to be grammatically correct. Ensure that the word opted is acceptable for that specific sentence. Continually build meanings of the sentences then select the right one. Generate alternative solutions then evaluate and select an alternative after this implementation.

Complete answer:
Option A) I only: - Sclerites -> “Fleas are difficult to squish because they have sclerites.” This word makes the sentence right and grammatically correct as the word sclerites refers to a component of exoskeleton.
Option B) I & II only: - Sclerites & Tough spines -> “Fleas are difficult to squish because they have sclerites and tough spines.” These words create the sentence wrong as fleas don't have tough spines.
Option C) II & III only: - Tough spines & Resilin in their joints -> “Fleas are difficult to squish because they have tough spines and resilin in their joints.” These words create the sentence wrong as fleas neither have tough spines nor resilin in their joints.
Option D) I, II and III: - Sclerites, tough spines and resilin in their joints -> “Fleas are difficult to squish because they have sclerites, tough spines and resilin in their joints.” These words create the sentence wrong as fleas have sclerites, they neither have tough spines nor resilin in their joints.

Hence the correct answer is option ‘A’.

Note:Read the question carefully then opt the correct and acceptable answer.
About flea -> Flea is the common name of Siphonaptera. It includes around a pair of 2,500 species of little wingless insects that survive as external parasites i.e. of mammals and birds.