the correct choice that Thomas can use to prove that side AB is equal to side DC is:
db ≅ db
Therefore, option D is correct
To prove that side AB is equal to side DC in quadrilateral ABCD where the diagonals intersect at point T, Thomas needs to use properties that show a pair of corresponding sides are congruent.
Given that angle ADB is congruent to angle DBC and angle DBA is congruent to angle BDC by the Alternate Interior Angles Theorem, Thomas can prove that triangle ADB is congruent to triangle DBC using the Angle-Side-Angle (ASA) postulate if he can show that the included side, DB, is congruent in both triangles. However, since DB is a common side to both triangles ADB and DBC, it is already congruent to itself.
Therefore, the fact that DB is congruent to itself (DB ≅ DB) can be used in conjunction with the congruent angles to show that triangle ADB is congruent to triangle DBC by the ASA postulate. Once the two triangles are shown to be congruent, it then follows that the corresponding sides AB and DC are congruent (AB ≅ DC) due to corresponding parts of congruent triangles being congruent (CPCTC).
So, the correct choice that Thomas can use to prove that side AB is equal to side DC is:
db ≅ db