It is not entirely clear to me what you mean. This is my best guess:
rate(k)
Also, I would change your code a bit in a few places:
number=unique(B(:,1));
tn=numel(number);
initial=rand(1,3);
rate=zeros(tn,1);R2_value=zeros(tn,1);
ft = fittype('a*exp(-x/b)+c');
for i=1:tn
current=ismember(B(:,1),number(i));
[F,gof2] = fit(B(current,2),B(current,3),ft,'StartPoint',initial);
rate(i) = 1/F.b;
R2_value(i)=gof2.rsquare;
end
k=find(R2_value>0.7)
