1 Bit Register out of D-type Flip-Flop
For bipolar TTL parts like the plain 74xx series, or 74LSxx, the inputs will appear High, unless pulled Low by a low resistance - under 500 Ohms should do. So, in your circuit, the D input will be high at all times.

Since inputs source significant current, it is common practice to use a pull-up resistor of 5K or so to +5V, with a switch from the input to Ground, to ensure that the input will really be recognized as Low when it should be.

For CMOS parts (74AC, 74HC), the inputs are high impedance and if left unconnected will float randomly between High and Low - they must be pulled up or down as required with a 5K or more resistor.  This includes inputs on unused parts of the chip, like the second latch in the 74xx74 package.in your circuit