The commercial had promised bounce and shine instead her hair was as dull and limp as ever. Which revision correctly punctuates the sentence? Question 4 options: a) The commercial had promised bounce and shine; instead her hair was as dull and limp as ever. b) The commercial had promised bounce and shine, instead her hair was as dull and limp as ever. c) The commercial had promised bounce and shine; instead, her hair was as dull and limp as ever. d) The commercial had promised bounce and shine, instead, her hair was as dull and limp as ever.

Anything with a comma in front of instead is simply not correct. The sentence is too long and the change too abrupt to be handed with a comma. So B and D leave a little bit to be desired.

I don't think C is correct, but it is just marginally not so. If I saw it in a student paper, I'd probably ignore it. I don't think you need the comma after instead. I would pick a myself, but that does not mean I'm correct.

Don't be a bit surprised to see the answer is C. I think the comma after instead is too much, but that is a matter of opinion.

Instead is an adverb. Most of the time you need not emphasize adverbs. It's a very hard call.