The binder in case of synthetic sand used for moulding is bentonite and water.

1.Binder : Binder is added  in moulding sand and provides strength to the moulding sand and enable it to retain its shape as mould laxity. Binders can be either inorganic or organic substance. The inorganic group includes clay sodium silicate and port land cement etc. In foundry shop, the clay acts as binder which may be Kaolonite, Ball Clay, Fire Clay, Limonite, Fuller’s earth and Bentonite. Binders included in the organic group are dextrin, molasses, cereal binders, linseed oil and resins like phenol formaldehyde, urea formaldehyde etc. Organic binders are mostly used for core making.

2.Synthetic sand :It is an artificial sand obtained by mixing relatively clay free sand, binder (water and bentonite) and other materials as required.

Hence, the correct option is (D).