TTL (Transistor-Transistor Logic) gates are digital logic gates that use bipolar junction transistors to implement logic functions. These gates are widely used in digital circuits due to their simplicity, reliability, and low power consumption.

The basic function of a TTL gate is determined by its internal circuit configuration, which is designed to perform a specific logic operation. In the case of TTL gates, the correct answer is option 'D', NAND (NOT AND) gate. Let's understand why.

NAND Gate:
- A NAND gate is a digital logic gate that produces an output that is the complement (inverted) of the logical AND of its input signals.
- A NAND gate has two or more input terminals and one output terminal.
- The output of a NAND gate is HIGH (logic 1) if any of its input signals are LOW (logic 0), otherwise, the output is LOW (logic 0).
- In other words, a NAND gate performs the logical AND operation followed by an inversion (NOT operation) on the result.

Explanation:
- TTL NAND gates are constructed using multiple bipolar junction transistors in a specific configuration.
- The internal circuit of a TTL NAND gate consists of transistors connected in a way that the output is pulled down to a LOW state (logic 0) only if all of its input signals are HIGH (logic 1).
- By using suitable biasing and transistor configurations, the output can be inverted to produce the complement of the AND operation.
- This configuration allows the TTL NAND gate to perform the logical AND operation with an additional NOT operation to produce the desired output.

Why not other options?
- Option 'A' (AND gate) is incorrect because a basic TTL gate does not directly implement the logical AND operation without additional components or circuitry.
- Option 'B' (OR gate) is incorrect for the same reason as option 'A'. A basic TTL gate does not directly implement the logical OR operation.
- Option 'C' (NOR gate) is incorrect because a basic TTL gate does not directly implement the logical NOR operation.

In conclusion, the basic function of a TTL gate is a NAND gate, which performs the logical AND operation followed by an inversion (NOT operation) on the result.