The $pK_a$ of a certain weak acid is $4.0.$ What should be the [salt] to [acid] ratio, if we have to prepare a buffer with a $pH=5$ using the acid and one of the salts?

Question

$$(a)\;4:5 \qquad (b)\;5:4 \qquad (c)\;10:1 \qquad (d)\; 1:10$$