uncompleted definition of "adopt"

[Zishuli]

Here is a question in #TOEIC/ M150

2. v. adopt

(a) to find out; to come across; to get information about something
(b) to decorate; to imitate; to mimic; to copy an example
(c) to hire as a worker; to activate; to operate; to use
(d) to form a relationship with another person

The answer is (d). Of course it's acceptable but I think the given definition of it's is rather vague, or I'll say it's uncompleted.

I have looked up the dictionary and got the explanation as below:

. to bring (a person) into a specific relationship, esp to take (another's child) as one's own child.

[Gray]

Hi Zishuli,

I agree with you, that choice 'to form a relationship with another person' was vague.
'take into one's family' is closest to the meaning.

~Gray
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I love words, sounds envy me.
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First lesson - English, not english. I, not i. ~A student of English

[Esl_Expert]

I agree. The definition provided is not particularly good. The test also ignores other definitions of the word "adopt".