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The instrument used for setting out an offset at right angle, is called as:

The instrument used for setting out an offset at right angle, is called as:
1). open cross-staff
2). French-cross staff
3). adjustable cross-staff
4). optical square

This Question has 2 answers.

The instrument used for setting out an offset at a right angle is called an **optical square**.

An **optical square** is a precise instrument used in surveying to establish right angles for offsets. It works based on the principle of reflection and is more accurate compared to other types of cross-staffs.

Thus, the correct answer is:

$\boxed{4}$ **Optical Square**

French cross staff

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