The correct answer is option 2: Mohorovicic.

The Earth`s crust and mantle are two distinct layers that make up the Earth`s interior. The boundary between these two layers is known as the Mohorovicic discontinuity, or the Moho for short.

Option 1: Gutenberg is not the correct answer. The Gutenberg discontinuity, also known as the Lehmann discontinuity, is a boundary between the Earth`s core and mantle, not between the crust and mantle.

Option 3: Conrad is not the correct answer. The Conrad discontinuity is also known as the Conrad discontinuity zone or the Conrad zone. It is a boundary within the Earth`s crust, not between the crust and mantle.

Option 4: Repetti is not the correct answer. There is no known discontinuity named Repetti that separates the Earth`s crust from the mantle.

In summary, the correct answer is option 2: Mohorovicic, as it is the discontinuity that separates the Earth`s crust from the underlying mantle.