If the DNA strand has nitrogenous base sequence ATTGCC, the mRNA will have?

The correct option is "UAACGG".

To understand why this is the correct answer, we need to look at the process of transcription, where a DNA strand is used as a template to synthesize a complementary mRNA strand.

During transcription, RNA polymerase reads the DNA template strand and pairs each DNA nucleotide with its complementary RNA nucleotide according to specific base-pairing rules:
1. Adenine (A) in DNA pairs with Uracil (U) in RNA.
2. Thymine (T) in DNA pairs with Adenine (A) in RNA.
3. Guanine (G) in DNA pairs with Cytosine (C) in RNA.
4. Cytosine (C) in DNA pairs with Guanine (G) in RNA.

Let us apply these base-pairing rules step-by-step to the given DNA sequence:

$$\begin{array}{rcl}\text{DNA base}A& \to & \text{mRNA base}U\\ \text{DNA base}T& \to & \text{mRNA base}A\\ \text{DNA base}T& \to & \text{mRNA base}A\\ \text{DNA base}G& \to & \text{mRNA base}C\\ \text{DNA base}C& \to & \text{mRNA base}G\\ \text{DNA base}C& \to & \text{mRNA base}G\end{array}$$

Combining these complementary bases in order gives the mRNA sequence: UAACGG.