If abc=adc, which is true by cpctc ?

If ΔABC = ΔADC, then B. BC = DC by CPCTC.

CPCTC is an acronym for Corresponding Parts of Congruent Triangles are Congruent. 

Answer choice C and D are not appropriate answers because they are not the corresponding angles of the triangles. 
Answer choice A is not the appropriate answer because although the statement is true, CPCTC would not be the correct reasoning.

The next number is 31.

3 + 1 = 4
4 + 2 = 6
6 + 3 = 9
9 + 4 = 13
13 + 5 = 18
18 + 6 = 24
24 + 7 = 31