Questions

No improvement required. Grant means to permit as a favor. As the interview will be a favor the use of Grant is correct.

PMVVY (Pradhan Mantri Vaya Vandana Yojana) pension limit extends to March, 2020.
The Union Cabinet, chaired by Prime Minister Narendra Modi approved extending the investment limit from Rs 7.5 lakh to Rs 15 lakh under the Pradhan Mantri Vaya Vandana Yojana (PMVVY).
It also extended the last date for a subscription to the scheme till March 31, 2020. The time limit was earlier supposed to end on May 4, 2018.
The PMVVY is being implemented through Life Insurance Corp (LIC) to provide social security during old age and protect the elderly aged 60 years and above against a future fall in their interest income due to uncertain market conditions. The scheme provides an assured pension based on a guaranteed rate of return of 8 percent per annum for 10 years.

Muscle cramping is a common problem encountered by athletes and non athletes alike. Defined as painful involuntary skeletal muscle contractions, cramps may be categorized as either non exercise related or exercise related. Traditionally, such cramping was believed to arise from dehydration, electrolyte imbalances (including magnesium, potassium and sodium),accumulation of lactic acid, or low cellular energy levels.

Calcitonin is 32-amino acid linear polypeptide hormone that is produced in humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimopharyngeal body. It acts to reduce blood calcium, opposing the effects of parathyroid hormone.

The error lies in the first part of the sentence. After superlative adjectives, majorly two prepositions are used: of and in. We use in with a singular word referring to a place or group. Eg: I am the happiest woman in the world. We use of before plurals, before time periods such as year/month. Eg: Sunday is the best day of the week. Here, in this case, the subject is the 'school', thus, it should be 'in the school'.