In Indo-Islamic construction, _________and lime mortar was an important factor in construction.

Correct option is D

The correct answer is (d) concrete.
• In some cases, concrete and lime mortar were used in Indo-Islamic construction, although it wasn't as prevalent as stone.
Incorrect Options Explanations:

•  stone: Stone was widely used, but concrete became more prominent in certain phases of Indo-Islamic architecture. 
•  mud: Mud was used in earlier constructions but not in combination with concrete.
• wood: Wood was mainly used for doors and roofing, not as a primary construction material.

Information Booster:

Important Indo-Islamic Structures:

• Qutub Minar: Built by Qutb-ud-din Aibak.
• Red Fort: Built by Shah Jahan.
  
• Taj Mahal: Built by Shah Jahan.

Concrete Use:

• Concrete was first used in Indo-Islamic architecture by Sher Shah Suri during the construction of the Grand Trunk Road.