The problem with English. It is a living language. In contemporary usage A,B,C, and D would all be considered natural usage, in spoken English.

We find ourselves as slaves to rules that no longer apply. There is a good article here www.theguardian.com/[...] about grammar rules that no longer apply.

As an aside I am currently watching a BBC documentary series made almost 50 years ago, the use of language is very different to how we would speak today.

I am also curious about the source of the questions. Is it an old text booK?

Here endeth my first and final free English lesson.

this guy's got not one but two grammar threads going. very nice.

look at the OPs profile. lots of threads asking for grammar and language stuff AND a classifies offereing tutoring services. pwerhaps we is answering his students questions.

Dazzler, what's the difference between answering me and my student? What's wrong with keep on learning?

Being a teacher doesn't mean I know everything, it only means I need to know more.

Being a teacher doesn't mean I know everything, it only means I need to know more.

haha ... Dazzler.
nothing wrong with it. we just want our cut.

Haha, I totally accept all the mistakes I made, without them I could not go further

Usually when the sentence has words indicating it's in the past "enjoy-ed", the time is best kept the same for other verbs within that sentence.

Technically you can use all 4 options nowadays but A: "seat-ed" would be the best fit.
Alternatively B: "Sat" is good too, but people tend to use "While sat comfortably". as it's more descriptive of being/happening "at the same time".

In addition I have another example:
"What's wrong with keep on learning?"

"Learning" has the ending "-ing", requiring the "keep" to become "keep-ing".
It is of course normally it is subject to which person (I,you,he,she,it,we,they) you are using, but not here.

This is something else and relates to a different use of words all together:
"Without them I could not go further".
Making progress in this instance is not a motion*, meaning you use "get" instead of "go". "you get further".
If you use "go further" in this sentence it would mean you'd feel ashamed / not up to the task.
* it's a state of mind.

This above is a simplified explanation :)
If anyone wants to contribute, feel free to do so :)

As @culture opines - A. Seated is the correct answer, grammatically.

SAT - implies someone put her in the seat - as though she were a child, but the sentence does not support that contention. Alternatively - to use SAT one would have to complete the sentence as, "She sat ...", but again - as @culture notes - that conflicts with the remainder of the sentence's verb tenses. As with Chinese - English language students must pay attention to the context (relationship) of the word, within the entire sentence.

SITTING - implies an ongoing action, which conflicts with the rest of the sentence - however the nuance is subtle - so SITTING is also a potentially correct answer, but as @culture notes - the tense of the verb (sitting) conflicts with the other tenses <enjoyed> <closed>. Mixing tenses in written english is a stylistic faux pas, but is quite common in colloquially spoken English - doesn't make it right though.

I forgot the form - but is "seated" past perfect tense?

@culture...touchée