The correct option is
C In
The word, 'in' is a preposition which expresses the situation of a noun being enclosed or surrounded by something. The phrase, 'in the sun' means surrounded by sunshine all around. The word, 'under' means 'at a lower level than'.
When we talk about being outdoors in the sunshine, the phrase, 'in the sun' is used. The phrase, 'under the sun' is normally used as an idiom. It emphasizes that there is a large number of something or 'as many as you can imagine'. Eg: 'I have tried all the possible solutions under the sun, but unable to solve the problem'.
Hence, option C is correct and option A is not correct.
The word, 'below' means 'at a lower level or layer'. The word, 'below' is used to talk about the level of something on a flat plane. If there is a star in the sky at a lower level than the sun, we would say, 'The star is located just below the sun'. However, the expression 'below the sun' cannot be used to describe the position of someone standing on the earth. Hence, option B is incorrect.
The word, 'into' is a preposition, which means 'to or toward the inside of (something)'. Eg: 'She walked into the room'. This preposition, 'into' is not used along with the verb, 'stand'. The phrase, 'stand into the sun' does not convey any meaning. Hence, option D is incorrect.