"What do you think of her?"
Translation:Vad anser du om henne?
"tycka" for opinions. The meaning of this sentence is : What is your opinion about her?
"tänka" for reflections, thoughts. Or intention of doing sth. "Vad tänker du på?"="What are you thinking about?" ; "Jag tänker på dig" = "I think of you"
"tro" = for "believes" ; "jag tror" = " I think (but I am not sure) "
tänka på is 'think about' as in 'having something on one's mind'. So you can't combine it with the question word what.
tänka om is 'think about' as in 'having thoughts about', 'think [something] about', and in this case, the thoughts have a content, so you can ask what you think.