Answer:
B. 26amu
Step-by-step explanation:
The nucleus of an atom in an element contains protons, electrons and neutrons. The proton number equals the electron number in a neutral atom. The proton number is also called the atomic number, denoted by Z.
The mass number also called Atomic mass unit (amu) in this case, of an element is the sum of the proton number and the neutron number in the atom of that element. The mass number is denoted by A.
Hence, A = Z + N
Where A = Mass number
Z = Proton (atomic number)
N = Neutron number
In this question, N is 11 while Z is 15
Hence, the mass number of the element in the question, in AMU, is 15 + 11 = 26amu.