Safa berjualan kue secara online, pada bulan pertama rata-rata keuntungannya sebesar Rp 75.000,00 pada bulan kedua sebesar Rp 125.000,00 begitu seterusnya menurut barisan aritmatika. Besar keuntungan Safa selama setahun adalah...

The profit in the first month ($a_1$) is Rp 75,000.00.
The profit in the second month ($a_2$) is Rp 125,000.00.
The common difference ($d$) is the difference between consecutive terms:

The question asks for the total profit over a year, which means 12 months. So, $n = 12$.

The sum of the first $n$ terms of an arithmetic sequence is given by the formula:

Substitute $n=12$, $a_1=75,000$, and $d=50,000$ into the formula:

The total profit for Safa during a year is Rp 4,200,000.00.

The calculated total profit is Rp 4,200,000, which corresponds to option C.

The final answer is $\text{C. Rp 4.200.000}$.