Answer:
B. -1.5
Explanation
ELASTICITY of DEMAND is the responsivenes of demand to change in its factors (good price , related goods price, income , taste)
CROSS ELASTICITY of Demand refers to demand change due to 'related goods price' factor. It is positive in case of Substitute goods (having direct relationship) , negative in case of Complementary (having inverse relationship)
FORMULA = %change in demand / % change in price
In this case : %change in flank steals demand / %change in gas grills price
= -15/10 = -1.5 [Complementary Goods]