Assertion: The watches having hours hand, minutes hand and seconds hand have a least count of 1 ms.
Reason: Least count is the smallest value that can be measured accurately by an instrument.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
(C) Assertion is correct but Reason is incorrect
(D) Assertion is incorrect but Reason is correct

Hint
Least count is the minimum measurement that is possible using an experimental instrument. For example, the least count of a measuring tape is 1 mm.

Complete step by step answer
Least count is an important parameter in the experimental operations. It is the smallest value that can be accurately measured by an instrument. Any value lower than this cannot be measured by the said instrument. For example, take the general rulers that we use for stationery. The minimum distance we can measure using the scale is 1 mm. This is the least count. But if we try to measure a speck of dust that is smaller than 1 mm, we can only guess its size. Our measurements cannot be relied on, in this case. This means the Reason is correct.
Now, we will check if the Assertion is correct. A watch with the hours, minutes and seconds hand moves and shows time as the seconds hand progresses. The smallest unit of time we can measure using this clock is the second, which is its least count. Hence, the assertion that the least count of this clock is 1 ms is incorrect.
Thus, the correct answer is option (D): Assertion is incorrect but Reason is correct.

Note
Since most of the instruments are machine-produced, they can and may have some errors. This error mostly shows up in the form of least count error, which is either slight overestimation or underestimation of its value. For example, sometimes you might notice the electronic weighing machines show some value even when nothing is put on it. This the least count error, and it must be subtracted from the apparent readings to get the true weight of the object placed on it.