The answer is B. 1/3+1/4
To do this, you have to put them into the same denominators. Do this by finding the common factor of both denominator. Then after that multiply the numerator by the number that it takes for the denominator to be multiplied to reach that common factor.
A. 6/8+1/4
The biggest common factor that 8 and 4 have is 16. So this is not the answer, since the denominator has to be 12.
B. 1/3+1/4
The factor that the both have in common is 12.
1*4=4 3*4=12. 4/12
1*3=3 4*3=12. 3/12
4/12+3/12=7/12