The word, 'diligent' is an adjective which means 'having or showing great care in one's work or duties'.
The word, 'rapid' is an adjective which means 'very fast'.
The word, cut-throat' is an adjective which means 'fierce and intense'.
The word 'sparse' is an adjective which means 'in short supply'.
The adjective which can most appropriately describe 'competition for electoral seats' is 'fierce and intense'. Hence, option C is correct.
The adjectives 'showing great care', 'very fast' and 'in short supply' cannot describe "competition for electoral seats'. Hence, options A, B and D are incorrect.