int a = Mercury;
int b = Venus;
int c = Earth;
int d = Mars;
int e = Jupiter;
int f = Saturn;
int g = Uranus;
int h = Neptune;
int q = quit;
int selection;
int weight;
int speed;
should be..
1 2 3 4 5 6 7 8 9 10 11 12
string a = "Mercury";
string b = "Venus";
string c = "Earth";
string d = "Mars";
string e = "Jupiter";
string f = "Saturn";
string g = "Uranus";
string h = "Neptune";
string q = quit;
char selection;
int weight;
int speed;
and this if statement
elseif (selection == 'a' || 'A')
should be elseif (selection == 'a' || selection == 'A')
along with the rest of your if's.
Hopefully, this will get you on the path to correcting your program.