Removing all the non essential attributes so that it looks something like this:
<iframe width="100%" height= "100%" src= "https://forms.office.com/Pages/ResponsePage.aspx?id=abC........." style= "border: none; width:100%; height:100vh"> </iframe>
Also make sure the iframe containing/parent tag's width and height is big enough to allow the form to be displayed. This usually happens when the width or height is to small. The form I used had a min-width and min-height of 320px. Setting the parent tag to have a min-width and min-height of 320px solved the problem.
Sample:
<div style="width:100%; height:100%; min-width:320px;min-height:320px">
<iframe width="100%" height= "100%" src= "https://forms.office.com/Pages/ResponsePage.aspx?id=abC........." style= "border: none; width:100%; height:100vh"> </iframe>
</div>