Answer:
The correct answer is option (E) Federal regulations once prohibited developing television programs in conjunction with the marketing of toys, but such programs are.
Step-by-step explanation:
Letter A is incorrect because the relative clause which makes the sentence confusing.
B is also incorrect because the verb at the end is singular while the subject is plural.
Option C is incorrect because television programs required for thriving.
Letter D is also incorrect because the subject they gives the sentence an ambiguous meaning.
Option E is the correct answer because the connectors and verbs are used properly.