I usually give feedback.  One never knows when they might have to work with that agent, and it never hurts--never- to play nice.

a. if I have anything to report

b. and c. if I do not have anything to report

The best feedback is an offer.

I always give feedback, I learn at a young age in Real Estate, you give and people will give feedback back

Depends:
IF we are seriously considering an offer . . . "We are reviewing several properties" no way am I tipping my hand to the listing agent
IF there is something listing agent needs to know, like I found the door not locked when I arrived . . . gonna read them in
IF what they really want is an agent to give an honest assessment of price, condition etc to help listing agent manage their seller . . . I'll give it to them UNLESS we are considering the property.

I submit but carefully, since I do not want to compromise my client on anything they may be interested in. 

As Candice expertly said, you have to watch what you say. I am faster with it if client is not interested.

Always reply with comments.

When feedback or input is requested, I remain a servant in all matters

It's rare to get prompted for feedback out here.

it depends on how detailed the feedback form is and other factors. most of the time A, sometimes B.

ALWAYS ! ALWAYS ! ALWAYS ! Provide Feedback ! Even if a simple no. It is one of the Responsibilities to other Agents if you show their Listing. as well as common courtesy, in our business of relationships, the ones with other Agents is a long term commitment.

I always, always, always provide feedback. My listing clients are always looking for feedback on showings of their homes? Why would I not extend that courtesy to listing agents when I've shown their homes. It's part of paying it forward. The only time I might not provide feedback is if it's a request from an agent who never provides feedback to me. And then I send feedback saying "I'd be happy to provide feedback on his/her listings when they decides to send me feedback." It works. We had one agent in Charlotte who would never provide feedback. Right after I sent him that message which was automatically forwarded to his client... he started providing feedback to me when he showed my listings.

Normally A, but I watch what I say if the buyer is interested.

I do not get feedback requests nor do I request them. 

I submit if asked.

It depends! "A" if it is a good prospect. "C" if it is a dog. And, "B" if it is an agent that is obnoxious.

A.

It depends. Sometimes buyers aren't ready to disclose what they like or dislike about a property. They might decide to take a 2nd look, so if I do reply to feedback requests, the answers will be vague and basically meaningless.

depends on the agent requesting, do they give me feedback?  If I have given them feedback 10 times and they ignore all my requests for feedback they go on the ignore list.

a. I definitely leave feedback even if only to say it doesn't work. 

A. 

I submit right away as a courtesy and always hope that I get similar speedy responses. 

 Right  MLS uses “Showing Time” and it will pester you forever for feedback 

( so you may as well do it the first time they ask ) 

I know how much it means to MY sellers, so always give feedback.

I always submit feedback if requested. I may not answer all questions, it depends on how interested my client is. So many agents do not bother and decline to provide feedback. I've found it makes me memorable. More than once did a listing agent call to thank me, and, more than once, did it set the stage for offer negotiations. 

I give hones feedback and it helps down the road so A