Internal cork of apple fruit develops due to the deficiency of
A. Magnesium
B. Iron
C. Manganese
D. Boron

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Hint: Growth and development requires a sufficient amount of nutrients. Deficiency of any nutrient leads to several disorders. One example is the internal cork of apple which leads to the formation of lesions in the internal region of the fruit and affects the quality of fruit.

Complete answer: Boron deficiency in apples lead to the development of internal cork. Minimal quantity of boron is required by plants. It is a trace element that can cause disorders in case of deficiency. It is deficient in waterlogged soil and causes the following symptoms in plants:
a) Internal cork: This symptom can be seen after 6-8 weeks and can only be seen when the fruit is cut and not on the external surface. Round marks appear on the flesh, spots dry up and the colour of spots turn out to be darker from the centre in the outward direction, spongy lesions are produced and cavities can also be formed in the core region and a brown tissue is formed which is mostly in the core region.
b) External cork: It occurs due to boron deficiency in early growth stages. It leads to the formation of brown spots which crack and dry up. Brownish or greenish lesions having green coloured borders are formed, which later on dry up.
Symptoms are seen on a leaf and stem when boron is deficient. Leaves turn yellow and new shoots die. If left untreated, spring dieback develops which leads to the death of buds and twigs. Deficiency of boron leads to rosette leaves. Leaves become shortened and brittle and thick.
Hence, the correct answer is option D.

Note: With few exceptions visual symptoms of nutrient deficiencies are seldom seen in woody landscape plants. Except for highly alkaline soils, where iron and manganese will be unavailable, leaf colour generally looks normal.