Which of the following 8085 microprocessor instructions performs HL = HL + DE?

Concept. In the 8085, DAD rp is a 16-bit "double add": it adds a specified register pair to the HL pair and leaves the sum in HL (HL ← HL + rp). The operand rp is encoded by a single letter that names the high register of the source pair — B for the BC pair, D for the DE pair, H for the HL pair, and SP for the stack pointer. Only the Carry flag is affected; the sign, zero, parity and auxiliary-carry flags are left unchanged.

Application. We need HL ← HL + DE, so the source pair must be DE. Selecting the source pair step by step:

• The target accumulator pair for DAD is always HL — that matches the required destination.

• The required source pair is DE.

• The letter that encodes the DE pair is "D", so the instruction is DAD D, which executes HL ← HL + DE.

Cross-check (contrast the other forms). Substituting each other source letter shows none of them adds DE:

• DAD B → HL ← HL + BC (adds the BC pair).

• DAD H → HL ← HL + HL (doubles HL).

• DAD SP → HL ← HL + SP (adds the stack pointer).

Hence DAD D is the unique instruction that performs HL = HL + DE.