The 2nd blank answer choices, when considered, would render the usage of all the options incorrect, except options (b) & (d). The second blank needs a noun, whereas option (c) and (e) are verbs and (a) does not fit the context. Prevalent (means widespread) does not provide a coherent meaning to the sentence with 'presence' in the second blank, rendering option (b) incorrect. 'Most striking' is correct, making option (d) the correct choice.