Which bond connects two amino acids?

Identify the correct matching pair from the following.

The peptide chains loop,twist and fold back on themselves due to the formation of . . . . . . bonds to form a stable tertiary structure of protein.

$A$ lubricant,mucin in saliva,is made up of:

Based on which group do the properties and types of amino acids differ?

Which of the following amino acids contains a polar and positively charged '$R$' group?