What is the length of $TU$ if △PQR is congruent to △STU?

Two figures are said to be congruent if and only if they have the same shape and size.

Two triangles are said to be congruent if all three corresponding sides are equal and all three corresponding angles are equal in measure.

Given: $∆PQR\cong ∆STU$

By CPCT rule, corresponding parts of congruent triangles are equal.

Then, $PQ=ST, QR=TU, PR=SU$

and $\angle P=\angle S, \angle Q=\angle T, \angle R=\angle U$

Thus, $QR=TU$

$\therefore TU=6 \mathrm{cm}$     [∵ QR = 6 cm]

Therefore, option B is the correct answer.