length of t and x_bp are not same. Theymust have same length to plot the graph using plot command.
You may use maximum available points to plot by modifying your code as
% Generate a lowpass signal
fs = 1000; % sampling frequency (Hz)
t = 0:1/fs:1; % time vector (s)
f1 = 10; % frequency of the signal (Hz)
x = sin(2*pi*f1*t); % lowpass signal
% Perform FFT and IFFT
X = fft(x);
X_bp = [zeros(1,100), X(101:401), zeros(1,499)]; % create bandpass frequency domain signal
x_bp = ifft(X_bp); % Create a bandpass filter
fc = [5 15]; % bandpass cutoff frequencies (Hz)
order = 50; % filter order
b = fir1(order, fc/(fs/2)); % FIR filter coefficients
% Apply the bandpass filter
x_bp_filt = filtfilt(b, 1, x_bp);
% Plot the results
subplot(2,2,1); plot(t, x); title('Lowpass Signal'); xlabel('Time (s)'); ylabel('Amplitude');
subplot(2,2,2); plot(t(1:900), x_bp(1:900)); title('Bandpass Signal (Frequency Domain)'); xlabel('Time (s)'); ylabel('Amplitude');
subplot(2,2,3:4); freqz(b); title('Filter Frequency Response');
