Introduction:
In electrical engineering, relays are widely used devices that control electrical circuits by opening and closing contacts electromechanically or electronically. They are classified based on various factors, including their constructional features. This question is asking us to identify the type of relay that is not based on constructional features.

Moving Iron Type Relay:
- Moving iron type relays have a movable iron armature and a fixed coil.
- The armature moves when the coil is energized, and this movement is used to open or close the contacts of the relay.
- These relays are commonly used for AC circuits and are suitable for both high and low currents.

Static Relay:
- Static relays do not have any moving parts.
- They use solid-state components such as transistors, diodes, and operational amplifiers to perform the relay function.
- Static relays are faster, more reliable, and have a longer lifespan compared to electromechanical relays.
- They are commonly used in applications where high-speed operation, precise control, and immunity to mechanical wear and tear are required.

Induction Relay:
- Induction relays use the principle of electromagnetic induction to operate.
- They consist of a coil and an iron core that create a magnetic field when energized.
- The magnetic field induces eddy currents in a conducting disc or a cup-shaped rotor, causing it to move and operate the relay contacts.
- Induction relays are commonly used in protective relaying applications, such as overcurrent and distance relays.

Current Relay:
- Current relay is not a specific type of relay based on constructional features.
- It is a generic term used to describe relays that operate based on the magnitude of current flowing through them.
- Current relays can be of various types, such as overcurrent relays, undercurrent relays, differential relays, etc.
- The constructional features of a current relay can vary depending on its specific application and design.

Conclusion:
Based on the above discussion, the correct answer is option 'D' - Current. Current relay is not a specific type of relay based on constructional features, but rather a general term used to describe relays that operate based on current magnitude. The other options, moving iron type relay, static relay, and induction relay, are all specific types of relays based on their constructional features.