The random sample comes from a normal instead of a uniform sample (assuming U stands for uniform). Also all the data you generated is in the (a,b) interval which will give you 1 for question a).
nsamples=100; % number of samples
n = 225; % sample size 225
a = 1;
b = 2;
data = -3+9*rand(nsamples, n); % random 100x225 data sample from a uniform (0,1) distribution
cMean = mean(data, 2); % mean of each sample
answer_a = sum(cMean > a & cMean < b)/nsamples; % how many data means are between a and b
answer_b = mean(cMean);
answer_c = std(cMean, 1);
hist(cMean);
title('Generating 100 Samples of Size 225 Simulated U(-3,6) Random Numbers Histogram Question 1 Part d)'); %title of histogram
xlabel('Z'); %labeling x-axis
ylabel('Frequency'); %labeling y-axis
