Consider a graph G with $12$ vertices. Assume that it has a unique spanning tree. Consider the statements:

$1.$ G is a tree

$2.$ G is a complete graph

$3.$ G is a connected graph

$4.$ G is an acyclic graph

Which of these statements are true?

a$)1,2,3$

b$)1,2,4$

c$)1,3,4$

d$)2,3,4$