The table below shows the total utility data for products $x$ and $Y.$ Assume that the prices of $x$ and $Y$ are $$3$ and $$4,$ respectively, and that consumer income is $$18.$

$\backslash$table$[[$Units of $x,$Total Utility,Units of $Y,$Total Utility$],[1,9,1,16],[2,15,2,28],[3,19,3,36],[4,21,4,40],[5,22,5,42]]$

Refer to the above table. What is the level of total utility for the consumer in equilibrium?

a$.51$

b$.43$

c$.47$

d$.61x$