"Is it possible that I can put all the six textbox in the same position of the respecitve graph?"
Yes. Here's an example that places each text object at Position [0,1] in 'normalized' Units, which is the upper-left corner of the axes.
f = figure();
tl = tiledlayout(f,2,3);
names = "("+string(char('a'+(0:5).'))+")";
for ii = 1:6
nexttile(tl)
plot(randi([1,100])*rand(1,10))
text(0,1,names(ii), ...
'Units','normalized', ...
'VerticalAlignment','bottom', ...
'FontWeight','bold')
end
