This work relates to image segmentation. you can do it in multiple ways, for perfection results you have to try with different methods. I suggest you read the Image Segmentation chapter in Digital Image Processing (Gonzalez Book). You can apply different ways, local thresholding or global thresholding depending on your required results (read about Otsu's Thresholding technique)
The Easiest way you can proceed as followings-
gray_im=rgb2gray(imread('055_edited.bmp'));
subplot(121);imshow(gray_im) % Decide the threshold as requirement of your results
threshold1=gray_im>100;
subplot(122);imshow(threshold1);
% Perform some morphological operations to remove all extra parts
