Medical experts have amassed evidence concluding that users of smokeless tobacco
be more prone to heart disease, hypertension, and mouth cancers than people who do not use smokeless tobacco.
A) be more prone to heart disease, hypertension, and mouth cancers -> If we had "recommending", which is a bossy word, I would love to go with A. However, "concluding" is not a bossy verb. We need "are" instead of "be". Incorrect.
B) are proner to heart disease, hypertension, and mouth cancers -> proner, we need more prone. Incorrect.
C) suffer heart disease, hypertension, and mouth cancers at a higher risk -> Now, we are comparing "higher risk", it is not the intended meaning of the sentence. "more prone" is suitable for the comparison. Incorrect.
D) experience heart disease, hypertension, and mouth cancers at heightened rates -> We lost the comparative word. It is important to see non-underlined part has "than", which gives a hint of comparative word. Incorrect.
E) are more prone to heart disease, hypertension, and mouth cancers -> This is better.
So, I think E.