Whether it is a lattice? Why or Why not?
In this diagram every pair has GLB and LUB.
It is not a lattice, because the pair $\{d,e\}$ has no least upper bound; it has upper bounds $a,b,c$ but no least upper bound.
Likewise, the pair $\{b,c\}$ has no greatest lower bound.
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It is not a lattice, because the pair $\{d,e\}$ has no least upper bound; it has upper bounds $a,b,c$ but no least upper bound.
Likewise, the pair $\{b,c\}$ has no greatest lower bound.