Assume there were x voters in the city .
0.4x promised to vote for P(Two-fifths of the voters promise to vote for P) out of which 0.06x voted for Q(15% of two-fifth i.e. 15% of 0.4x)
0.6x promised to vote for Q( rest promise to vote for Q) out of which 0.15x voted for P(25% of rest i.e. 25% of 0.6x)
Total votes P got = 0.40x - 0.06x + 0.15x = 0.49x
Total votes Q got = 0.60x + 0.06x - 0.15x = 0.51x
As given in the question that P lost by 2 votes
0.02x = 2
the total number of voters is : 100
Hence Option A is the Correct Answer.