input argument q is undefined

function trapezoid(f,p,q,n)
h =(q-p)/n
sum = 0;
for i = 1:n-1
     x = p+i*h;
     sum = sum+ 2*f(x);
end;
total = (f(p)+f(q)+sum)*h/2;
fprintf('The value of integration is: %8.4f',total)

[KALYAN ACHARJYA]

Read here

[Steven Lord]

Show/tell us how you call your trapezoid function.

FYI, you should avoid creating variables with the same name as functions in MATLAB. If later on you wanted to call the sum function inside the trapezoid function, you would not be able to do so, as the name sum refers to your variable.