I need a help please!

B. KBr  + F2

Explanation:

In displacement reactions (reactions in which an element replaces another element in a compound) like these, reactive element displaces the less reactive element.

A. NaBr  +  I2  

This reaction will not occur because Iodine is not able to displace bromine from the compound.

B. KBr  + F2

This reaction will occur because flourine is more reactive and hence is able to displace bromine from the reaction.

C. LiF  +  Cl2

This reaction will not occur because chlorine is not able to displace flourine from the compound.

D. NaCl + I2

This reaction will not occur because Iodine is not able to displace Chlorine from the compound.

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.