In that question, 'How' means 'In what way?'. The answer to 'How do you think of the plan' would be 'Constantly', 'optimistically' - any possible adverb (often one that refers to a state of mind*)
PS This probably doesn't apply to Korean, so you can ignore it keannu. But some readers may be interested to know that in the Romance languages this (the mind) is what, historically, was the basis of the standard mechanism for forming adverbs: initially, it worked only with adjectives that referred to mental states - placida mente is Latin for 'with a placid mind'. But more recently, in French, Spanish, Italian and so on, adding -ment[e] turns any adjective into an adverb.
The phrase 'How can you think' is commonly used in rhetorical questions. 'How can you think of marrying again so soon after she died?'