Option B: 'Very' means 'of a high degree, extremely'. It relates to the given sentence: He was extremely interested in small tricks during his childhood.
Hence option B is correct.
Option A: 'Too' also refers to a high extent, but it has negative connotations: He was too sick to work. The given sentence is affirmative and 'very' is more suitable.
Option C: 'As' expresses a likeness or similarity: 'to be as light as a feather'. The given sentence does not draw a similarity between two things.
Option D: 'So' expresses purpose or reason. The pair 'so...that' expresses a high degree, along with the result of that high degree: she was so hardworking that her boss gave her an extra leave.
The given sentence does not have a clause beginning with 'that'.
Hence these options are wrong.