Draw all histograms and put them in a container with the class hist-all.

x = faithful$eruptions
for (i in n_breaks) {
  hist(x, breaks = seq(min(x), max(x), length.out = i + 1),
       main = paste('n = ', i))
}

But we hide all of those images initially with CSS.

/* hide all images */
.hist-all img { display: none; }
/* but show the image with the class 'show' */
.hist-all .show { display: block; }

When the choice in the select input is changed, show the histogram with the selected number of bins.

(d => {
  const s = d.getElementById('n_breaks'),  // the select input
     imgs = d.querySelectorAll('.hist-all img');  // all histograms
  
  // add the class 'show' to the i-th image, where i is the
  // selected number in the select input
  function showImg() {
    const i = s.selectedIndex;
    imgs.forEach((el, k) => {
      // k is the index of the image; i is the index of the choice
      // in the select input; add the class 'show' if the two
      // indices are equal
      el.classList.toggle('show', k == i); 
    });
  }
  
  // show the initial image
  showImg();
  
  // show a different image when select input is changed
  s.onchange = showImg;
})(document);