I have seldom seen my mother______pleased with my progress as she is now. <something> as pleased as now. This sentence is about a comparison. How pleased? As pleased as she is now. "as pleased as now" is the base, the thing compared with. She is AS pleased as now. "So" seems unlikely: "as" seems much more likely. "so" does have a sense of comparing sometimes. very, too & rather have no comparison. She is very pleased. She is too pleased. She is rather pleased. These are complete without anything else. These have no element of comparison: as pleased as now. As pleased as <something>. I suspect that "so pleased as now" is also correct, as the others are not correct. To me though, "as" works best.

I think the answer to this question is A. as ... as & not so ... as

i guess A is right so....as~~~固定搭配~~~

Right, "A" is correct. If the sentence were without "as she is now", you could have used "A", "B" and maybe even "C" But in this case "A" is implied. However, theoretically it is possible to say "I have seldom seen my mother very/ too pleased with my progress as she is now" You know, everything is possible in speech! But first you should study the rules to know where to violate them. And the rule is that when you compare things, usually you use the construction liput mentioned